The Doctor sprinted awkwardly towards the TARDIS, which he had left parked at the end of the driveway.
"Doctor! Doctor! Where are you going?"

Arthur came dashing out of the house after him, stumbling awkwardly on his slippers. The Doctor simply ignored him, smashing into the door as in his haste to get inside he had forgotten that he had locked it.

"What are you doing? It's just a box."

But the Doctor was already inside and at the control panel, looking at the readings on the moving display.

Arthur called from just outside the door.
"Doc- why is it echoing like that? Doctor? Oh..."

His voice broke off as he pushed open the door to reveal the control room inside.
"It's -"

"-bigger on the inside than on the outside, yep. Another dimension. But these readings, they're strange. It's almost like...but it can't be."
Arthur took a single step inside, as though he believed the whole thing to be a hoax and that he was about to come up against an invisible barrier, or a computer screen, or the back of the police box.

"It can't be what? What is it?"
The Doctor stopped fiddling with the various instruments on the central column and turned to face Arthur.

"This is an alternate universe. I'm not supposed to be get here."

Whatever it was Arthur had thought the Doctor might say, it clearly wasn't that. His jaw went slack with shock, his eyes widening in an almost comic fashion.

"An alternate universe? They really exist?"

The Doctor nodded.

"Yes. But they are supposed to be sealed off. Travel between them is not supposed to happen. It can't happen. Which means there is a gap, a tear or a weak point between a point in your universe and a point in mine. But why? What caused it?"

Arthur was doing his best to keep up, and the Doctor had to admit, that he was doing a fairly good job.

"A tear? Where?"

The Doctor walked passed Arthur and exited the TARDIS, the other man following closely behind.
"I'm not sure. It could be anywhere."

The human was clearly not impressed by the lack of certainty.

"Great. That's great. What are you doing? What is that?"

Arthur was indicating to the bulky metallic object in the Doctor's hand.
"Sonic screwdriver. And I'm taking readings. Trying to get a hold of what stuff this universe is made of."

A glimmer of recognition passed over Arthur's face, but almost as soon as it had appeared it was gone.

"Isn't every universe made of the same stuff?"

The Doctor looked geniunely caught off guard by the question.

"No. Why would it be?"

Arthur didn't seem to have an answer to this.
"Um...oh. Right. So is this universe's...stuff the same as yours?"

The Doctor held the screwdriver in front of his face, gazing at a small digital panel on it's side.

Arthur's eyebrows shot up.
"No. Which is very not good news."

The Doctor resumed his apparently random waving about of the screwdriver.
"Why is it very not good?"

He examined the display again, decided that the readings were accurate and pocketed the screwdriver before facing Arthur directly.
"Last time I came across an alternate universe, the physical laws were identical. But not this time."

Arthur's face was serious and he was obviously concentrating very hard on what the Doctor was saying.
"What do you mean? Gravity is different?"
"Well, yes. But not because of any intrinsic property of the matter in your universe as such, and not just the visible matter. It's an extrinsic property, of visible matter, dark matter, in, out, and raw matter."

As he spoke, the Doctor's hands were flailing wildly about his person, and he started to bounce up on the balls of his feet.

"In matter?"

As though unable to stand still any longer, the Doctor began to pace back and forth in front of Arthur.
"Yes, in matter. As I was saying, it isn't the structure that's different, it's the energies that are. Dark energy, light energy, cold energy, warm energy. It's all different here. Which then alters the way that matter interacts. Which means I shouldn't be here for too long."
"Why not?"

The Doctor stopped, scrutinising Arthur as though he was trying to read his thoughts. He leaned in close, squinting slightly as he gazed directly into the dark blue eyes.
"Because if I stay here for too long, the difference in the fundamental energies of the matter in my body and the alternate gravity, electromagnetism, and other critical forces will tear my body apart. And if it doesn't tear my body apart, it will start tearing everything around me apart. Similarly, everything near the tear will start to disintigrate, and then everything around it, until the fabric of the universes themselves will be unable to hold itself together against the upsetting alien energies and will cease to exist, vanishing into the time vortex."

With the last sentence the Doctor had once again began to pace, much faster and more vigorously than before, as his voice grew thick with excitement.

"So you're saying if we don't find and repair this gap..."

A look of realisation was slowly dawning on Arthur's face.

"Both my universe and your universe will collapse into a singularity and disappear. Yes."

A sideways grin etched itself across the Doctor's face, an appearance of genuine enjoyment mixed with a ghost of fear.

The fear on Arthur's face was not a ghost, but pure and unrestrained. The Doctor was nodding encouragingly, as a teacher would to a student who was getting everything correct.
"Big oh. Oh. Oh!"

He clapped his hands together and rubbed them gleefully as his pacing increased further still in speed.

This time the Doctor did not stop, but continued his frenetic movements.
"The tear."

The curiousity was bursting out of Arthur now and he was leaning slightly forward as a subconscious indicator of how badly he wanted to know what was going on.
"The tear?"
"Around it! The space around it Arthur! Don't you see? If we can find the part of your universe that is disintigrating, we can find the tear!"

As he spoke, the Doctor turned in circles, his arms waving about above his head as a way of emphasising the word 'around'.
"Oh. Well that should be simple. After all, the universe is tiny and all that."

The sarcasm was bitter and sharp, and alerted the Doctor to the major problem in finding the tear. The vastness of space.
"Good point. I need some time to think."

And without another word, the Doctor crossed the lawn and went back inside Arthur's small house.

For the next hour the Doctor sat on the sofa, fiddling with his sonic as though it might suddenly change the readings he was getting and tell him everything he had ever wanted to know. Arthur sat silently watching him, attempting to comprehend what he had just been told and wondering why, oh why, did these things always happen to him? Yet again, he had thought that maybe, just maybe, he had a fairly decent grasp on what his place in things were, just to have it completely shattered by an alien. Finally, unable to take the silence and the self reflection that came with it, he turned the TV on. It was a news bulletin, the words Breaking News: Cardiff, screaming at him from the bottom of the screen. A female news presenter with the curliest blonde hair Arthur had ever seen was on for a moment before it cut to footage of a location that Arthur could not recognise.
At around 3pm this afternoon, a large section of Cardiff, Wales disappeared. This is untouched footage of the aftermath. The Doctor was leaning forward in his seat, his face eager and keen. As you can see, most of the centre of the city has gone. This is not a hoax. Those are the outer suburbs of Cardiff, once up to 20 minutes drive from each other, now neighbours. Where the inner city has gone, and how, remains a mystery. This is the...
A smile had spread across the Doctor's face and settled there.
"Arthur Dent, I think we've found the tear."