"You want me to use my mind, but you don't give me enough information," Sherlock said, rushing after the Counsellor as she stepped out of the TARDIS. It only took him two seconds to figure out where they were: an Underground station, specifically, Piccadilly Circus. He looked behind him at the TARDIS and found that it had assumed the form of a broken Oyster card machine console, set just off the wall. He smiled. The Counsellor's vehicle was amazing, and she was right; it would be disastrous for that technology to fall into the wrong hands. Or any hands.

"I gave you all the information I have the patience to give you right now." She strode up the steps out of the Underground station into the heart of Piccadilly Circus.

"The faces!" Sherlock protested. "I can't begin to figure this out without more information about the faces. It's impossible. You say they're all the same man. Impossible."

"As impossible as my TARDIS?" she asked, turning on him. She studied his face carefully and sighed. He could feel some of her warm breath on his face and he felt suddenly calmer. He frowned. Definitely cheating.

"I can't do my job this way."

She nodded. "Fine. Let's walk. Take my hand. I'll explain as we move."

He balked. "Wait –"

"I'll turn off the visual part of it until we're stopped," she said. "You will have to see it, though."

He took her outstretched hand in his and heard her voice in his head as they started walking. It was rather pleasant, like listening to an audiobook:

It probably won't surprise you to know that Time Lord physiology is different from human physiology. Outwardly we're identical, but there are a few significant differences that made us doubt it was even possible to interbreed with humans. Of course, the Doctor changed our conclusions on that front.

Even so, we are different. Time Lords have two hearts. Our blood contains traces of the time vortex, which makes it possible for us to connect with our TARDISes and understand the deeper physics of the multiverse. And finally . . .

The Counsellor had stopped moving and tugged sharply on Sherlock's hand. They were standing next to the fountain, and there were tourists all around them, arms circling each other, faces tilted up to cameras and mobile phones, smiling broadly as they captured the moment. She pulled a mobile out of a coat pocket and turned it on them.

Close your eyes, she said in his mind. He did.

Images then – he sensed he was back on her home world, and the war was raging around them. He could feel the ground tremble under the force of the explosions. He looked up in time to see a giant slab of some strange material – something between metal and rock – fall directly on to him (her?) and pin him. He felt blood, his (her?) blood, pouring out of his mouth and nose.

"God," he gasped. He felt like his lungs were collapsing and he was underwater and there wasn't enough air. "I'm . . .dying."

The images continued as Sherlock stood in Piccadilly Circus, next to the fountain, a smile frozen on his face, his eyes closed.

We all do eventually, she said in his mind. But for Time Lords, the moment can take an eternity to come.

He felt himself rolling out from under the slab. The ground continued to tremble, but for the moment the disintegration of the space around him (her) was stabilized. It was safe enough.

His lungs were starving, but he was able to whisper, "Safe enough for what?"

A brilliant montage of images – family members, friends, people at the end of incredibly long lives suddenly erupting in blinding streams of orange energy. He felt his borrowed hand lift; he saw a few age spots on the back of that hand, neat fingernails, and those telltale silver strings – but more than that, he saw a strange orange luminescence glowing on those hands. He heard her voice then, not the strong, pure telepathic voice of their connection, but something earthier, more organic: "Oh. At last."

Then his awareness was swallowed by that orange light. The sensations rolling through his body were excruciating, both pleasant and painful, and that was just too much.

He pulled his hand away, and suddenly the toxic fumes, the thundering ground, and the overwhelming sense of pleasure and pain were gone. Not understanding what had just happened, he turned to the Counsellor and said the most illogical thing rolling around in his brain: "Did we just have sex?"

Her jaw dropped open and she searched his face for two seconds before she started laughing, loud, not-unpleasant peals of laughter ringing throughout the plaza. She stepped away from him, apparently helpless to do anything else, and doubled over, her hands on her knees as she let the laughter out.

"I think I asked a perfectly logical question."

She breathed deeply, still shaking with the remnants of her amusement. "I can read your mind, remember? I know better. That was the least logical thing you were thinking."

"Then why did I ask it?"

"Because you were panicked. You have a history of behaving illogically, irrationally, when you're frightened or panicked, don't you? You like to know what's coming and you like to think you can predict if you don't know." Her tone was very business-like now, and he found that unnerving. His heart was racing and he couldn't help himself – something about the switch between helpless mirth and calculating calm reminded him of –

I will burn the heart out of you.

Sherlock was shaking.

She took his hand again, and the full-body tremors torturing him slowly stilled. "Let's walk again," she said.

"I don't know if I can trust you," he confessed.

"Do you have to trust your clients?" she asked.

"If my client is transporting me around in a time-travelling spaceship, I think a certain level of trust should be established."

"Good point." She switched her monologue to her inner mind. I know you won't know the significance of this oath, but I swear by my blood, by the pattern of Kasterborous, and by my secret name that I will return you home, to your place and time, when this is all over. You are safe with me, Sherlock Holmes.

He thought again that he could leave, just walk away from this – but then he wouldn't know.

Her voice resumed. What you saw was my one and only regeneration. He saw very quick, very brief images of two faces, both female, both different: one had a silver-blonde mane of pin-straight hair, cool violet eyes, and a very aristocratic nose that gave her an undeniably haughty air. This was not the other face, the face he knew: tawny hair, full and wavy, warm bronze eyes, and a sensual face that made her seem pouty, not proud.

"Regeneration?" he asked.

She answered in her mental voice: When a Time Lord is near-death, our bodies release copious amounts of a unique hormone called lindos. This prompts the third strand of our DNA to begin the change from our current form to our new form. Our biomechanical processes freeze and the whole of our bodies are transformed, even to the sound of our voices. You wouldn't recognize us. It's impossible.

"So you're saying that this husband of yours has gone through this at least four times, maybe more."

She nodded. "Possibly up to the limit, if the limit even exists after what he's done."

Sherlock shook his head. "Impossible."

"You keep using that word. I don't think it means what you think it means."

He smirked. "I've seen that film, you know."

"What film?" she asked, apparently sincere in her confusion.

He waved it away with his free hand. "Doesn't matter. The point is, if I can't even be sure what he looks like, how can I be sure we've found your Doctor?"

She smiled. "You'll know."

"But how?"

"I'll know."

"I know I'm starting to sound extraordinarily repetitive, but I can't do my job."

She leaned closer to him, and he was struck by the smell of her. It wasn't a shampoo or perfume. It wasn't a regular, organic odor. What he smelled was like fire, like electrical circuitry and volcanic ash.

He pulled away, but she closed the distance again, his hand firmly held in hers.

"I think you can."