Title: Relative Genius

Writer: pacejunkie

Characters: Ten, Donna, Albert Einstein

Rating: PG/K+

Word Count: 8,522

Summary: The Doctor is thrilled to meet Albert Einstein, until an experiment gone wrong threatens to absorb all of the life force on earth unless these two geniuses can put their heads together and stop it.

Writer's Notes: The Tenth Doctor and Donna belong to the BBC. Einstein was a real live person and belonged to himself. Various internet sources including Wikipedia were used for historical research.

His career might have been slowed in its tracks, but Albert Einstein's mind refused to stop. He was only twenty-six, yet he felt like a man who had lived a long hard life, struggling every day to make sense of the world around him. Everything he saw caused his thoughts to churn, running at a pace far beyond those of his peers, moving in directions that were entirely new. Some would call it mania or insanity, but to Albert it was simply awareness, and he had been aware for as long as he could remember.

He recalled how at the tender age of four, his father had showed him a pocket compass. Young Albert realised then that there must be something causing the needle to move, something existing in the otherwise apparent "empty space." There was no such thing as empty space, he knew, and he set out to prove it. By twelve he had mastered Euclidean geometry and had moved on to infinitesimal calculus. By sixteen he was performing his first thought experiments on the speed of light.

It was as though Albert himself moved at that very same speed.

Yet when he graduated from the Polytechnic in Zurich, Switzerland in 1900 with a degree in mathematics and physics, he spent two long years searching in vain for a teaching post. He had never experienced anything like it. Nothing had ever slowed him down before, yet here he was discovering that perhaps the rest of the world failed to move as quickly as he did. In the meantime, the ideas kept coming, but their outlets were limited.

It was a family contact that finally secured him a position at the Federal Office for Intellectual Property in Bern, as of all things, an assistant patent examiner. All the while, he quietly published his papers and continued with his research into the nature of light and matter not because it was his job but simply because he was compelled to. It was that which gave his life substance and meaning, sustaining him as much if not more than the food and necessities that his salary provided. Thus, Albert found himself craving those hours when the work day ended and his true work began.

In truth, he lived for the time in between - the empty space – where things mattered.

"Herr Einstein," said his assistant Matthias Hauser, the young man's arms laden with a large wooden crate of assorted odds and ends, "I've run the experiment on the first lot. Shall I start next on these?"

Matthias was barely twenty but bright and eager. At first he had insisted on calling Albert Professor until his employer finally convinced the young man that Herr Einstein would do. The title would come later, Albert assured him, when the world managed to catch up to him. In truth he was not a man of affectation; he would have been fine with Albert, but the nervous intern was eager to show respect and he had no wish to traumatise the poor boy by insisting that they be treated as equals. Two years later, Matthias had proven to be most capable, though the Herr Einstein nomenclature refused to budge.

Albert looked up from his own work transcribing equations from his notes to one of several blackboards around the functioning home lab.

"Yes, yes, Matthias," he nodded. "Remember to note the frequency and intensity of the light you are using."

As he had been doing for several years now, Albert was experimenting with the properties of light. This particular experiment was designed to study what was known as the photoelectric effect, in which the application of light caused electrons to be emitted from matter. They were studying all forms of matter – solids, liquids and gases – as they were affected by all forms of light. Matthias had been extremely helpful in this regard. Having an extra pair of hands to run the experiment meant that Albert could focus on his writings, supervising the assistant as needed.

Matthias set the crate down and removed a bit of scrap metal, setting it in place on the table in front of him. It was an odd bit of scrap; small, no bigger than his closed fist. He first set to describe it in his notes, but had difficulty identifying the precise material. It was much too heavy to be tin or even iron; it felt more like lead but displayed none of the common properties of that substance. The colour was strange too. It was not quite grey, not quite green, not quite brown, yet depending on how it was turned, it appeared to be all of these at the same time. As for its shape, it contained grooves and notches as though it were hand tooled. It might have resembled a machine part, but Matthias couldn't imagine the machinery.

His thoughts were interrupted by the sound of the doorbell.

"Ingrid is still shopping," said Albert by way of explanation, referring to his housekeeper. He put down his chalk, "Keep working, I'll see who is at the door."

When Herr Einstein left the lab Matthias switched on the light and began taking notes. His head was bent low over the paper, his fountain pen in hand, and thus he failed to see the small shimmering blue cloud of gas escape from the chunk of metal as the light hit it.

The gas hovered for a moment, sensed a nearby energy source and then buried itself into the back of Matthias Hauser's head.

Matthias reached up to scratch an itch and yawned at the same time.

The Doctor was bouncing from one foot to the other on the landing, like he was standing on hot coals. He could barely contain his energy at the best of times but today it seemed to Donna as though he'd shoot right off into space without his TARDIS if he let himself go.

"Would you stop," said Donna, as they came to the door, "You're making me nervous."

"Have I thanked you yet Donna?" he asked her. "Really, thanks for this, this is brilliant."

"Well, it's the least I could do after the white sand beaches of Kolhaar," she told him. "You were sporting about it, but I could tell you'd rather be someplace else so I thought you'd like to choose the holiday for a change."

Donna looked at the name on the mailbox, checking to be sure they had the proper address before ringing the bell. She had travelled with the Doctor long enough to know anything was possible but still she could hardly believe her own eyes when she saw it: A. Einstein, as plain as day.

"I might have known you'd pick something like this though," Donna sighed as she reached out for the bell.

The Doctor was too busy grinning from ear to ear to notice the slight. As they waited he began extolling the virtues of the most famous physicist of the twentieth century.

"Albert Einstein is the greatest human genius ever," crowed the Doctor, "This is 1905. It became known as Einstein's miracle year of discoveries. He publishes four revolutionary papers in this year alone that form the groundwork for his general theory of relativity which he'll complete by 1915."

"So I shouldn't mention anything about E equals mc squared then?" joked Donna.

"No," said the Doctor, "Let him work that one out on his own, I have no doubt he will."

Donna looked around her at the lovely old European residential block on what was in fact a rare gorgeous late winter day. It wasn't her choice of a trip but she could make the best of any situation.

"At least I'll get to see the Alps while we're here," said Donna, "Switzerland's lovely."

Just then the door opened and there was a pause as the man at the threshold spied the unfamiliar callers. It took all of the Doctor's strength not to pounce, but he contained himself and instead thrust out a hand.

"Mr. Einstein, I presume," the Doctor said with a smile.

"Yes?" the man replied, offering his hand in return more slowly.

He looked much younger than Donna expected. In the famous photos he always had that frizzled grey hair. But here it was still brown and neatly combed back. The Doctor took his hand and shook with such enthusiasm that Donna feared he would take the man's arm clean off.

"Oh this is brilliant, sir. Brilliant!" the Doctor enthused.

"I'm sorry," replied Albert, "But you are…"

"Right!" the Doctor said, "Sorry! I'm the Doctor and this is Donna Noble. Thrilled to meet you sir, absolutely thrilled. Well, I say thrilled, more like blown away, excited beyond belief, 'is this really happening somebody pinch me' sort of thrilled…"

Donna cleared her throat and sang in a low undertone, "I think you're frightening him."

For his part Albert appeared bemused and intrigued, though quite confused.

"Well," the Doctor began again, "Actually, I've read your paper on the existence of atoms and just had to meet you."

"Are you a scientist?" asked Albert.

"Well, yes in fact," nodded the Doctor, "sort of… almost… not quite…The truth is I sort of… dabble. But I'm clever, and I would love to discuss your work with you."

At this Albert beamed, "Well, come in, come in, out of the cold, the both of you. A part time scientist, I know the feeling. To be quite honest I am surprised to be known by anyone outside of a university, but someday we will both be recognized, yes?"

They followed Albert in through the foyer of his home and to a large room to the left that appeared to have once been the drawing room. It had been converted into a lab, with tables and bits of equipment set up everywhere and piles of papers. There were blackboards lining the walls full of equations that Donna couldn't make sense of. On a stool at one of the tables was a young man, propped up on his elbows, taking notes. He put his pen down and looked up when they came in.

"This is my assistant Matthias," said Albert.

Donna smiled and said hello but the Doctor's attention was drawn elsewhere, or more precisely, it was drawn everywhere else. He gaped around the lab like Charlie Bucket in the Wonka factory. Approaching the first blackboard, the Doctor reached into his pocket and pulled out his glasses.

"This is it," the Doctor said with wonder, "it's your special theory of relativity. You've worked it out."

"Yes, but I haven't published it yet, I'm working on the paper now," Albert said, "How do you know?"

"Oh, like I said, I'm just clever," said the Doctor with a sniff, "I knew you were getting there, I could see it in your previous works."

"Yes," said Albert with enthusiasm, "I have found that regardless of where you stand or the speed at which you move, the speed of light remains the same. Always the same! It is constant, never changing. Isn't that marvelous?"

"Oh yes, it's brilliant," said the Doctor, eyes still fixed on the board like he was studying the Mona Lisa, "Isn't it brilliant Donna?"

"Yeah, fascinating," said Donna, with a bit less sincerity.

For her part Donna wandered the lab, allowing the Doctor his visit before they were off to the Alps for some late season skiing. She had no interest whatever in all of this science babble but the Doctor was so happy that it gave her some vicarious pleasure just to see him.

"I am glad of your visit Doctor, and that you appreciate my work," said Albert, "I often feel quite alone here."

"Oh, you have your assistant," said the Doctor.

"I'm sorry, not here," Albert explained, waving his hands around the room. Then he pointed to his head, "Here. Here I have been alone my entire life."

"Ah," said the Doctor, understanding, "Yes, I know, lonely at the top, quite literally. Geniuses often feel that way."

They returned to the equations and began discussing them in more detail than Donna could follow. On her own in the lab Donna turned to the assistant Matthias for some conversation. The young man was hunched over his papers rubbing at his eyes and seemed to need a break.

"Hello," she said, "I'm Donna."

He looked up from his work, lids slightly heavy. Must have been at it for hours, Donna thought.

"Matthias Hauser," the young man said.

"So what's it like, Matthias," she began, "working for Albert Einstein?"

"Herr Einstein is brilliant," said Matthias, "I am honoured to work for him. We are doing research that will change the world someday."

"I bet you will," said Donna, smiling. "What about you then, are you a scientist?"

Matthias stifled a yawn and rolled his neck.

"Excuse me," he said, "Yes, I hope to be one day. I am studying physics at the University."

He yawned again and this time he could not contain it.

"Why so tired?" asked Donna, "One too many late nights revising, or does Mr. Einstein have you working too hard?"

"No," Matthias, explained, "My exams are still months away. My work here is only part time, it is not difficult. I help Herr Einstein run his experiments and take notes on my observations. He is an excellent employer."

"Well something's come over you all of a sudden, you can barely keep your head up," she said. Then Donna leaned in and spoke in a conspiratorial tone with a wink, "Look, I know all this science stuff is a bore but as it's your job you can at least pretend like you're interested."

Matthias smiled, shook his head and straightened himself up, "I am fine, but perhaps some air."

He rose to go to the window, took two steps and then his knees buckled. He fell forward and Donna caught him halfway to the floor with a surprised shriek.

"Doctor!" she called with the young assistant unconscious in her arms, "Mr. Einstein!"

The Doctor and Albert were deep into a discussion on electrodynamics when they heard Donna cry out. Turning together, they ran to her, crouched on the floor with a prone Matthias in her lap.

"What happened?" asked the Doctor.

"I dunno, we were just talking and he fell over," Donna explained, "He seemed exhausted, like he'd been up for days."

The Doctor glanced at Albert, but the physicist was stymied. "He was fine this morning," he shrugged, "Is he ill?"

The Doctor checked Matthias' pulse and then scanned him with the sonic screwdriver. As he did, Albert leaned in close, examining the strange tool from inches away with a furrowed brow. Donna could tell that his brain was working furiously, formulating questions he would soon ask, but the Doctor proceeded as though he hadn't noticed.

"His pulse is a bit slow," the Doctor said as he scanned, "If it was exhaustion it would be racing. I'd say it's more like sedation. It's as if he's sleeping."

"But we had coffee together this morning, he was not like this," protested Albert, still eyeing the screwdriver, clearly torn between his curiosity and his concern for his assistant.

"When I spoke to him he could barely hold his head up," said Donna. "What could come over him that quickly? Was he drugged?"

"That's odd," said the Doctor.

He adjusted some settings on the sonic and scanned again.

"What's odd?" asked Donna.

"These readings," said the Doctor as he continued to scan. "He's not merely sedated, he's being drained. The amount of energy his body should contain has been decreased by more than sixty percent. It's dropped another five percent just as I was scanning."

"Energy? What sort of energy?" asked Albert.

The Doctor let out a long sigh, "I suppose the best description would be life force. Every living being contains a life force, it's the energy that animates them, keeps their systems running, their brains active…"

"Like batteries," said Donna.

"Right," said the Doctor, "To use your metaphor, we recharge our batteries when we eat and sleep and if we didn't…"

"We'd drop," said Donna, looking down at Matthias.

"Something is feeding off of Matthias' life force," said the Doctor, "The condition he's in now it's as if he hasn't slept for a week."

"Will he recover?" asked Albert, growing concerned.

"Not if the drain continues," said the Doctor, "If it drops to below ten percent his brain function will cease and his whole system will shut down."

"You mean he'll die?" said Donna, alarmed, "What do we do?"

"If it began here then the answer is hopefully here in the lab," said the Doctor. He turned to Albert, "Is there someplace we can move him where he can be watched?"

"I have guest quarters on this floor," said Albert, "My housekeeper Ingrid will be back soon."

Carefully, the Doctor and Albert picked Matthias up and carried him to a bed in the guest room. Donna followed. As they finished, they heard the sound of the front door. A middle aged heavyset woman with her hair in a bun appeared in the doorway of the guest room carrying a cloth bag of groceries. Donna presumed this was Ingrid. Ingrid looked around the room at the strangers and at Matthias on the bed and then looked to her employer quizzically. Albert instructed her to put the groceries in the kitchen and return.

When she did, Albert told her, "Matthias is ill. Please watch over him and tell me if there are any changes."

"Yes, Herr Einstein," said Ingrid.

"And now Doctor, this instrument of yours," began Albert as they returned to the lab, "I am anxious to study it."

The housekeeper settled down into a chair after her employer had returned to his lab with the two strangers. All the while, Matthias slept soundly. After checking for signs of fever, Ingrid left the room for some tea. She returned a moment later with a cup and a book, sat back in her chair with her tea beside her and began to read.

The book was a romance novel, and Ingrid was lost in its pages. As she read and sipped at her tea, a blue shimmering light escaped from Matthias' body. It was drawn by the new life force it sensed, an abundance of energy now that the one it had been feasting on was nearly gone. The light was visible for only a second before it vanished into the back of Ingrid's head.

A few minutes later, Ingrid put her book down. She rubbed the back of her neck and got up to stretch. Reading always made her drowsy, and it wouldn't do to fall asleep when she was meant to be watching Matthias. She checked him again. He seemed a bit paler, his breathing barely perceptible on the back of her hand. She wondered whether she should tell Herr Einstein, and then she yawned despite herself. She would tell him, she decided, but first she needed a bit of fresh air.

Ingrid left the room and opened the front door. The crisp cold air woke her senses immediately and she felt a bit better. She stood on the landing for a moment and spotted the postman making his rounds. She would collect the mail and then head back inside.

"Good day," she greeted the postman as he came up the steps to where she stood.

The postman smiled and handed her the post. They chatted for a moment about the lovely weather. As they spoke, the blue light sensed a fresh energy source and floated from out the side of Ingrid and over to the postman. Ingrid returned to the house and the postman went on his way.

As he left, he yawned.

Ingrid stepped just inside and looked through the letters. There was one from her sister in Dusseldorf. She opened the letter excitedly and began to read, but the letters were blurring before her eyes as her lids got heavy.

The Doctor searched the lab.

"What was Matthias doing when he collapsed?" he asked Albert. "Was he working with any chemicals?"

"No. He was assisting me with my electron research," explained Albert. "It involved nothing more than light."

The Doctor looked at the materials scattered all over the workspace. "Where do you collect these things?" he asked as he sifted through them.

"There is a scrap yard outside of the city," said Albert. "I pay the owner a small amount and he allows me to search for bits of various materials. They are of no use to anyone else."

The Doctor picked up each bit of scrap in turn, examined each quickly through his spectacles and tossed it aside. When he came to the last piece he stopped.

"Oh. Hello," he said to it.

"What's that then?" asked Donna.

"It's just a bit of iron," said Albert, confused, "A machine part, no doubt."

"It may be a machine part but it's not iron," said the Doctor, bringing the scrap of metal up close to his nose and sniffing it. "It's arthrocite."

"Ar-thro-cite?" repeated Albert, spying the strange material as he tested the word on his tongue. "Fascinating. I've never heard of such a thing."

"That's because it comes from very far away," said the Doctor, turning it round in his hand, feeling the weight. "Thing is, it's a concentrated power source. Bounce light particles at the right frequency off of this and you'll tear a small hole in the universe. Well, I say a hole, a tiny one. A teeny tiny one, but big enough to let something through."

"Like what?" asked Donna, thinking of poor Matthias unconscious in the guest room.

Just then a heavy thud from outside the lab prevented the Doctor from answering her. All three of them snapped up and ran out to the foyer. There they found Ingrid. The housekeeper had collapsed just inside the door.

The Doctor leaned down and scanned Ingrid's body.

"She's alive," he declared, "but she's showing the same energy drain as Matthias."

"What's that there," said Donna pointing to the paper by her side, "Is that a letter?"

She reached down across Ingrid's prone body to get it. The Doctor touched Donna's arm to caution her.

"First the lab assistant, now the housekeeper, we don't know how this thing spreads," he told her.

"But I touched Matthias and I'm all right," said Donna, pulling free of his grasp and taking up the letter and envelope. She looked it over, noting the date. "She must have been outside collecting the post when it came over her."

"Doctor," asked Albert, "I think we should move her into the guest room with Matthias."

The Doctor looked at Donna and Albert and thought. Donna was right, Matthias fell straight into her arms and yet this thing hadn't drained her. For some reason it chose the housekeeper. Donna might have just been lucky, or it might have been a question of timing. Perhaps when Matthias had collapsed the thing wasn't quite ready to move on, not until later, when Matthias was nearly drained and it had encountered Ingrid. Perhaps it moved on when it required a new energy supply. If that was the case then it was best to move Ingrid now and not wait until her life force was depleted further or it might get hungry again.

"Okay, quickly," he said, and the Doctor and Albert lifted Ingrid and settled her onto the guest room sofa.

While there he checked on Matthias, scanning him. The young lab assistant was ashen grey.

"He's got only twenty percent of his life force remaining," said the Doctor grimly. "He hasn't got long."

He looked at Ingrid. The housekeeper's colour was better and she appeared only to be sleeping soundly. He knew it would be best to not hang about.

"We need to get back to the lab and find the source," he declared. "I'm putting them under quarantine. No one come in here until I can identify this entity and contain it."

He ushered Albert and Donna out of the room and reached for the door handle to pull shut behind him. As the Doctor closed the door he turned, and thus failed to notice when a shimmering blue light escaped from the fast depleting body of Ingrid and latched itself onto the most abundant supply of energy it had yet discovered in this strange world.

The light vanished down the back of the Doctor's collar.

The Doctor returned to the lab to examine the equipment more closely, shadowed even more closely by a curious Albert, who stared at the alien mineral in much the same way he had the sonic screwdriver. Placing the arthrocite in the centre of the experiment area, the Doctor turned on the light and scanned it as the electrons were released.

"I was right," he said, "There's a small anomaly in the space where the electrons separate off the arthrocite - a teeny tiny hole, teeny weeny in fact, barely perceptible, but its there."

He released the button on the screwdriver and turned to Albert and Donna.

"Something passed through here, from another reality into ours," he declared.

"Another reality?" asked Einstein, his eyes lighting up.

"Relatively speaking," said the Doctor.

Donna rolled her eyes at his dreadful joke and pulled the Doctor aside. Albert was so engrossed in his examination of the arthrocite that he failed to even notice.

"How did that bit of arthrocite even get here?" Donna whispered.

"Oh you'd be surprised," the Doctor told her, "bits and bobs go flying off of spaceships all the time. Most of the time they just burn up in the atmosphere but pure arthrocite has so much mass it could make it through. You ought to go digging through your average landfill sometime back in London. Some unscrupulous person who knew what to look for could make a fortune off of Intergalactic eBay."

"Intergalact…?" Donna began, incredulous.

They were interrupted by Albert, now watching them, clearing his throat. The Doctor and Donna stopped their conversation and turned around.

"Excuse me, but the things I have seen today…who exactly are you?" he asked.

"Just a genius," replied the Doctor, "like you, honestly. Well..."

"Relatively speaking, we know," said Donna. "Well maybe you two relative geniuses can put your heads together and work this out for Matthias and Ingrid's sakes."

"No worries, Donna, we're on it," replied the Doctor with enthusiasm and he bounded back over to where the experiment lay. "I'll know more when I've worked out what it was that came through, but my guess is its not malicious. A living entity not from this reality would need the life force of other living beings just to sustain itself. It could be acting on its own survival instinct."

"That means we probably can't communicate with it," said Donna, "but it may need our help to get back. Can we do that?"

"Oh yes," the Doctor declared.

The Doctor turned to the nearest blackboard and wiped it clean. "Sorry Einstein, I need this," he said as he began calculating equations of his own.

Albert moved closer and studied them but could barely make heads or tails of it. This unusual man appeared to be a physicist, chemist and biologist all in one. His equations borrowed from every known science including some he didn't recognize.

"Is this a calculation of the arthrocite's mass?" he asked the Doctor, pointing to a string of symbols. "And what is this symbol here?"

"That stands for the corresponding mass in an alternate universe," the Doctor explained quickly as he worked. "I need to work out the formula for this thing's own energy source from its reality. It's a bit tricky because there are a lot of variables and a few unknowns and some other bits I'm just guessing at…" He glanced at Albert who was looking at him as though he were speaking Xantholosian. "Sorry, you'll just have to trust me."

"I am a genius remember?" said Albert with a twinkle in his eye. "Just explain it to me."

The Doctor smiled. "Right, sorry."

He walked Albert through his equations step by step. Albert stopped him to ask carefully worded questions throughout, making it obvious to the Doctor that he was clearly grasping the basics. All the while the Doctor kept working, ever conscious that time was of the essence if they were to save Albert's assistant and housekeeper.

"So, basically," said the Doctor, concluding his lesson, "we need to create an environment that will mimic the energy from the entity's own universe and entice it back, like cheese in a mousetrap. Luckily this thing is still trapped in the house somewhere, so containing it all shouldn't be a problem."

"Brilliant," said Albert, "Absolutely brilliant."

Donna wandered over to the window as they worked, feeling a bit useless. She couldn't possibly imagine what she could contribute that would help. Their talk had gone way over her head ages ago. So she gazed out onto the sunny street, wishing they had gone to the Alps first. Trouble seemed to follow the Doctor wherever he went.

She sat on the window ledge and watched the postman make his rounds across the street. Many residents came out and met him at their gates, while others were already outside on this beautiful day, walking their dogs and pushing prams. The postman handed the letters over and moved along slowly, like he was dragging his body. One of the neighbours took his letters, sifted through them and stifled a yawn.

Donna thought of the letter that Ingrid had been reading when she collapsed and her mind began making connections. It began with Matthias and then moved to the housekeeper. Ingrid then met the postman and collapsed while still holding her letter. Then the postman went round making his deliveries to the entire street.

"Oh no," muttered Donna to herself, "It can't be."

But even as she said it the postman returned to his carriage and fell to his knees alongside it. Donna's head turned to the yawning neighbor. He had collapsed on his front lawn. Then a dog ran off down the street with its lead flying behind him, his owner having gone to sleep in the middle of the sidewalk.

"Doctor!" called Donna, "I think we're too late! This thing's already got out."

The Doctor raced over and looked out the window. Everywhere they looked people were dropping like flies. Donna couldn't imagine how she, the Doctor and Albert had managed to escape infection. She supposed it was just a stroke of luck that this thing latched on to the postman and left the house before sapping each one of them.

"How do we capture it now?" she asked.

"Now that it's out there we won't be able to lure it back," the Doctor said, "So we'll have to be a bit more forceful."

"What does that mean?" asked Donna.

"It means…" he began, but Donna could see that he was fresh out of ideas, "Back to the drawing board!"

He sped back to his board of equations and began erasing them all. When he was done he stared at the empty space and thought.

"Come on Doctor, think, think, think!" he muttered to himself as he slapped his head, "I could turn up the scent to reach a wider radius but I don't have the kind of power I'd need to cover the whole neighbourhood and if I miss so much as a particle this thing can still infect and drain the whole city in minutes. I have to make sure I get it all. A lure isn't strong enough, what I need is…is…"

"A vacuum," said Albert.

The Doctor spun around and stared at him.

"If we were to create a vacuum in the centre of the anomaly," Albert explained, "theoretically…would that work?"

"A vacuum," repeated the Doctor, "That's it! Brilliant! A vacuum… well, more like a Hoover actually. Negative space! We'll create a pocket of negative space that'll suck the entity back to its own universe. That should bring it back from everywhere it's gone, no matter how far it's gone, using only a fraction of the power. We can use the arthrocite as our power source. It should give us everything we need."

"Will everyone be all right then?" asked Donna, "Will they all wake up?"

"Well they can't take it with them, the entity I mean, the energy is incompatible with their reality," explained the Doctor, "so the life force they took should be restored, back to its source, or sources. Short answer - yes."

The Doctor picked up the chalk once again and started scribbling new calculations. Albert did his best to follow along and all things considered was doing quite well.

Halfway through, the Doctor dropped his writing arm for a moment and paused. He shook out his hand, took a quick deep breath and started again. He might have been imagining it but the chalk suddenly felt heavier in his hand. He looked at it, and as realization dawned, the Doctor knew he had to move faster.

"Negative space," asked Einstein, "That is similar to a singularity yes?"

The Doctor was writing furiously when Albert asked his question. The voice echoed in his ears, sounding far away. The Doctor paused and shook his head, blinking, before beginning again with his equation.

"Not exactly, but similar. It will have the same effect at least," the Doctor told him.

"Then I believe these figures ought to be reversed," Albert observed, pointing to the formula the Doctor had just completed.

The Doctor rubbed his eyes, and then realised that Einstein had spoken.

"I'm sorry, what?" he asked him.

"These figures here," Albert pointed at the board, "The mass must be calculated before factoring in the speed."

The Doctor stopped, and stepped back to look at what he had written. The numbers were blurring together. He knew what Albert had said made sense but he had thought he had written it that way. Now he couldn't even find the formula on the board. He looked for a minute longer anyway, pretending to examine his figures.

"Right…right…," the Doctor muttered, "Could you repeat that Einstein?"

Donna had been nearby the whole time, watching the Doctor work. At first she had thought it was her imagination but now she was sure. She leaned in close so only he could hear her.

"Doctor, are you all right?" she asked him.

"Fine, fine," he replied without looking at her.

He watched as Albert erased his formula and began rewriting it. Donna was unconvinced.

"You look like Matthias did before he dropped," she said.

This time the Doctor did look at her. His eyes were bleary and bloodshot and his breathing sounded laboured.

"I know," he admitted to her, "but I have to keep working. I have a lot more life force than a human so I should be able to hold out for longer. And if all else fails a pot of strong coffee would help."

Donna nodded, taking the hint. She went to the kitchen to make some.

Still a temp, she thought to herself as she went with a dose of good humour, fetching coffee.

Putting the coffee on the stove to brew, she thought of Matthias and Ingrid. Donna wanted to check on them, but resisted in deference to the Doctor's quarantine order. Instead she went past the guest room to the front door and looked out into the street again. The fallen looked more like corpses than before, pale and lifeless. Donna noticed that more people had since come along to offer assistance, only to join them. More were collapsing alongside the others even as she watched.

Ten minutes later Donna returned with the coffee and handed the Doctor a cup. He took it from her with his left hand and chugged it down while still writing. Then he handed it back to her for a refill. Donna poured and passed it back. Half of the second cupful went down the front of his shirt, but he didn't seem to notice. Donna poured again and again he drank.

"How's it coming?" Donna asked him.

"Nearly there," he said, "There's just one more thing to do to make it work."

As he scribbled his head lolled and he jerked it back up. Donna handed him a fourth cup.

But this time the Doctor couldn't lift it. It was like being handed a block of cement. The full cup dropped to the floor with a shatter of china.

"Oh sorry," the Doctor said, looking down at the puddle at his feet.

And then the Doctor did a perfect imitation of the cup as he too fell to the floor.

Donna gasped and she and Albert crouched down beside him.

"Doctor!" called Einstein frantically, "Your work is unfinished! What should I do?"

His eyelids were drifting closed. He felt like a spent candle, melting into the floor. Too tired to move, he struggled to get a few words out before giving in fully to the exhaustion.

"Reverse…the…polarity…" he murmured.

Albert looked to Donna, who was cradling the now unconscious Doctor in her arms.

"Well don't look at me," she said. "You're the genius."

Albert looked up helplessly at the formulas and equations the Doctor had been writing. Much of it was familiar but by no means all, particularly the bit at the end, and with each passing moment countless lives were at stake. The Doctor had said there was one more thing to be done, something that would transform a pile of useless calculations to a pocket of energy that would save the world, and Albert had to work out what that was.

"But…I've never seen anything like it," he protested, studying the board, "Reverse the polarity of what? What does that mean? And these equations here and here… they're nonsense."

"You're Albert Einstein," said Donna, "if anyone can work out the Doctor's nonsense it's you."

It was in his head now, floating in the ether as he slept.

A shared consciousness. Sentient.

Who are you, the Doctor asked it, reaching out with his mind.

We are Sensorii, it responded as one, and we are lost.

I can help, he said, but you're killing these people. You have to stop.

We have no substance here, the Sensorii replied. We are liquid in our world, we have form. Here we are nothing.

You exist even as atoms, argued the Doctor, Einstein proved that. You are not nothing. No one is nothing. Let me go and I'll get you back home.

But you contain so much energy. So much raw power. More than the others. It sustains us. If we release you we will surely die. We cannot.

Then we are both lost, said the Doctor. There's nothing I can do. We depend on Einstein now. Leave him alone and allow him to complete his work. He will save you.

And if he fails, the Sensorii asked, we will be forced to survive on this world and all of the energy it contains.

He's a genius, and the fate of humanity depends on it, the Doctor replied, he won't fail.

"I don't know if you can hear me Doctor but Einstein's working on it," Donna told him, "He'll sort it out and he'll save everyone."

Donna reached down and felt for the Doctor's pulse. Even at his usual double rate it was slowing, and the colour was draining away from his face. Donna rubbed at her head and wondered if she'd be next. Perhaps she should quarantine herself to protect Einstein, but then it didn't do the Doctor any good. What use was quarantine when they had no idea how this thing moved around?

If I'm infected it's too late anyway, she thought, best to stay here and try to be of some use for as long as I can.

Donna eased the Doctor back to the floor and stood, her stiff joints protesting.

"Is there anything I can do to help?" she asked Albert.

"Reverse the polarity…reverse the polarity…" Einstein muttered to himself as pulled at his hair, pacing in a circle. He returned to the board to study the Doctor's equations, wondering what he had left unfinished that he needed him to do.

It had to have something to do with the photoelectrons released by the light, he decided. That was the point at which they were trying to create negative space. In Albert's initial experiments, shining light upon the arthrocite caused electrons to be released, which had in turn pulled the entity from its own reality into ours. So reverse it, he pondered, and the entity should be drawn back. Therefore…

…reverse the polarity of the electrons

… and create a vacuum.

"I've got it!" Albert proclaimed.

He turned to Donna, finally acknowledging her, "I have a theory on how the Doctor's equations work. I think I can use them to create the pocket of negative energy that we need. And yes, I can use your assistance."

"What do I do?" asked Donna, eagerly.

Einstein led her over to the work area where Matthias sat earlier. The arthrocite was still in place under the lamp. He handed her a new light bulb, one that would give them the precise intensity they needed.

"Replace that bulb with this one. When I say so, turn it on and hold the light at this precise distance," he explained, showing her. He returned to the blackboard and consulted the formula again. "There is just one more thing…"

"What?" asked Donna.

"The arthrocite will power the vacuum but the Doctor indicated here that we require something called a 'sonic wave' to trigger it," he turned to Donna, "Sonic wave?"

"Oh! Hang on!" cried Donna.

She leapt up and returned to where the Doctor lay. Digging in his pockets she found his sonic screwdriver. She returned and presented it to Albert.

"Sonic wave," said Donna, "Trigger it with this."

Albert took the odd instrument in both hands carefully. "This tool creates sonic waves? Fascinating."

"I'm sure the Doctor would be happy to tell you all about it once we've saved his life," Donna prodded.

"Yes, yes of course," agreed Albert and he returned his focus to his work.

After a few more minutes the light was ready and in place. Donna held the lamp while Albert pointed the sonic screwdriver at the precise spot on the arthrocite where the light would hit it.

"Are you ready?" Albert asked Donna.

"Wait," Donna said, "Is this vacuum going to suck in everything in the room, including us? Should I be holding on tight to something?"

"No," Albert assured her, "According to the Doctor's calculations, it is designed only to attract what does not belong in this reality. We should not feel a thing."

"Then the only way we'll know if it's working is if everyone wakes up I suppose," said Donna. She glanced over at the Doctor, who looked as though he had stopped breathing completely. "We'd better hurry."

"All right then. 1…2…3," said Albert and he switched on the sonic just as Donna turned on the light.

Waves and particles joined forces as electrons were transformed into protons. They held steady, waiting for some sign. Donna held her breath and watched the Doctor. One minute passed, and then two.

He didn't move.

"Come on Doctor, wake up," prodded Donna.

Maybe the light wasn't strong enough, Donna thought, Einstein was only guessing at the Doctor's equations, he could have miscalculated any number of things. Matthias was already near death. If this didn't work they didn't have time to try again before people started dying. And if the Doctor died…

"Are you sure this is right?" asked Donna.

"No," said Albert, "But keep going."

Donna held the light in place as the sound of the sonic buzzed in her ear. Perhaps they had it on the wrong setting, she worried. The thing had dozens of settings, apparently, even though it only had one button. Of course there was also the possibility they were doing everything correctly and the entity was simply resisting while it gorged itself on the offerings here.

The buzzing was beginning to give her a headache. Albert was staring intently at the spot as Donna looked around the room for signs of anything. Her arms were going numb, while at the same time the heat of the lamp was burning her fingers. She was growing tired, most likely from stress and strain, she told herself. It was making the room hazy. A blue fog had started to form all around her, while at the same time, the sound coming from the screwdriver changed pitch abruptly.

The fog wasn't fatigue, she realised. It was them, the creatures or whatever they were. She looked over to where the Doctor was but could no longer see him through the gloom. The fog swirled and coalesced around where Donna and Albert stood at the eye of the storm. Albert realised it too and they looked at each other and smiled. Donna didn't dare let go of the lamp. She fought her weariness and held on, as the fog thickened and became a cloud that Donna swore was shrinking.

A short while later, the air in the room was clearing again, and Donna could see.

The Doctor was sitting up.

"Mr. Einstein," he declared, smiling from his place on the floor, "Did I ever tell you, you were a genius? Oh and by the way, you can stop now. Hole's closed and all's right with the world. The Sensorii are home. That's what they're called by the way, Sensorii. All in all, not a bad day's work, I'd say."

"We did it," cried Albert, putting down the screwdriver and embracing Donna, "We did it!"

Everyone in the street had been restored as had Ingrid and Matthias. The lab assistant and housekeeper awoke and joined them with no recollection of what had happened. The Doctor performed a quick and surreptitious scan and found their life forces to be intact.

"Come on, Einstein," said the Doctor, "You can walk us to our transport. I think you deserve a reward for saving the day."

Albert joined the Doctor and Donna as they left the house and went down the street.

The Doctor tossed the arthrocite up and down in his hand as they walked. "Tell you what, I'll get rid of this for you. Nothing but trouble, arthrocite."

"Where are we going Doctor?" asked Albert, looking around the streets for a vehicle. "Did you arrive by carriage?"

"Not exactly," said the Doctor as they approached the TARDIS. "But you'll like this, I promise."

He unlocked the door to the blue box and stepped aside, grinning. Donna stepped in first to demonstrate, beckoning to Albert, who had taken a small step back, confused.

"It's all right," said Donna, "Just a look."

Albert peered in and then moved forward. A step more and he was fully inside, gaping as they all did their first time, but there was also something more. Donna could practically see his mind working.

"No," Albert said finally, "impossible. Unless…"

"Unless?" asked the Doctor, entering behind them.

Albert approached the console and examined it. Most people didn't have the words to describe the TARDIS beyond bigger on the inside. But this was Albert Einstein, the Doctor reminded himself, and he most definitely knew what he was looking at.

"Relative dimensions!" Albert announced excitedly. "How did you manage it Doctor? Explain it to me."

"Well, it's quite simple really. Imagine two boxes, one over here and…" the Doctor began, and then stopped, "No, better not. Knowing you, you might just understand it and build one of your own and we can't have that. Just, keep working on your theories, you're doing fine."

"This…ship," Albert said, "It travels in space and it travels in time, I imagine."

"No flies on you, Einstein," said the Doctor.

"Then it is as I always suspected," he said, "Time is just another direction to travel in like space. I can complete my work on relativity."

"Oh, you have many more wonderful discoveries ahead of you," The Doctor assured him, "You should never stop questioning. The world will catch up to you someday."

"And you, Doctor," Albert replied with a smile, offering his hand in farewell.

"Not me," said the Doctor taking it and shaking, "I'll just keep outrunning it. As fast as I can."

Albert departed and the Doctor watched him go, thinking for a moment. Donna noticed as a melancholy look came over him. Then he quietly turned to the console and entered the coordinates for their next destination.

"So that's it then," said Donna, "I say we've earned ourselves a few days of skiing in the Alps, wouldn't you?"

"Yep," said the Doctor, "But just one quick stop first. Won't be a minute."

Berlin in 1933 was not the Germany Einstein once knew. Earlier that year Adolf Hitler had been appointed Chancellor and before anyone realised what had happened new laws were being enacted. The nation's half a million Jews were blamed for everything from Communism to the poor economy and hyperinflation. By April the first legislation was passed barring Jews from civil service employment, medical and legal professions and placing a strict quota on their numbers at schools and universities.

Despite his status at the University Albert knew the end was near. His wife Elsa had been fretting for months about the rising threat. But Albert still had important work to do. At first he had told her that he was too valuable to the Reich to dismiss. But as time went on, he was eventually forced to admit that no one was indispensable where the Nazis were concerned.

Certainly not a Jew.

He would be dismissed any day now and deported if he were lucky. Very lucky. In fact, he had heard rumours of worse things, plans the Nazis were making. The disappearances of his friends and colleagues lent credence to the gossip. These were not idle fears. This was no longer a safe haven, if indeed it ever was.

There was a knock at his front door and a moment later Elsa entered the room with an envelope.

"There was no one there, only this," she told him.

She handed Albert the envelope, her hands shaking. These days, a strange envelope delivered to one's door was rarely good news.

Albert smiled at her and opened it. In it were two travel visas and tickets for a ship to America, leaving tomorrow. After a moment he began to laugh. Elsa stared at him, waiting for an explanation for his odd behavior. Albert took her hand in his and grasped tightly, showing her the envelope's contents. Tears streamed down her face, tears of joy for the first time in months.

When she had gone to pack Einstein gripped the anonymous envelope and looked upwards.

"Thank you Doctor."