A man, lying in a crumpled heap on the floor of the loo, half-formed skin still sparking faintly like weak sunlight on the bottom of a pool. John Watson, hunched on the lip of the bathtub with his head in his hands, despairing.

Compelled by habit, he'd checked the man's vital signs right away. His breathing was shallow and his heart was racing, but he was alive and would most likely be fine. Fleetingly he'd considered moving him to bed, but that would mean accepting his identity, and he couldn't. This couldn't be Sherlock. It was too wrong.

And then Sherlock's last words had come back to him: Everything is going to change. Sherlock had expected this, that much was clear. He'd even apologised unprompted for freaking John out—a definite first. But what on earth was it that had actually happened?

And so here was John, sitting on the edge of the bathtub, trying to work out what he knew, getting nowhere, and reevaluating his life. He'd only meant to pause for a moment, but time wasn't passing normally today, and he still hadn't moved when he heard the teapot whistle. The sound was so utterly normal that it hurt.

The thought of the landlady finding him still here was enough to rouse him from his stupor. And he decided the least he could do was get this guy off the floor.

Several minutes later, covered in sweat, he heaved the body up onto Sherlock's chair just as Mrs. Hudson came by. Gasping for breath, he acknowledged her with a sidelong nod and sank onto his own chair. Human bodies were not light. (Neither, generally, are Time Lords.)

"Strange day, isn't it?" she said, looking at the white-haired man. She sounded a bit concerned, but not at all frightened. John was. He was terrified. Suddenly he felt inexpressibly grateful that he wasn't alone.

He sighed. "Sure, strange is one word. Bit of an understatement."

"How long d'you think before he wakes up? Hope you two can work this out."

John realised she believed without a doubt that it was Sherlock who sat across from him now. Was she too gullible, or was he in denial of an obvious fact?

"Well, I'm not waiting around," he said, standing, "or the tea'll get cold, you know." He slapped the stranger hard across the face.

No response. John checked for a pulse, just to be sure. It was there, but…Okay, that's bizarre. He'd never heard of a double pulse before.

"Something wrong?" Mrs. Hudson asked as he moved to unbutton Sherlock's coat. He put a hand on the man's chest where his heart should be. It wasn't there; it was further to the left. Then he moved to the right, because all he needed was more insanity today.

A few moments later, confused and defeated, he turned round. "He's got two hearts," he announced.

The landlady blinked mildly, thinking she'd simply misheard. He didn't blame her.

Two working hearts, in completely separate regions of the chest. And he'd checked, earlier, he'd checked and there had definitely been just one. But—of course it was the last thing he'd been worried about at the time—it had been left of normal, hadn't it? The whole cardiovascular system was fundamentally incompatible with anything seen in chordates, let alone a human. This was no chance mutation.

And the alternative to human was…what? An alien? The kind of alien whose humanoid appearance was a convenient coincidence for low-budget TV producers, or the kind that crawled out of a stolen skin to eat you?

Or both?

Frankly, it was a ridiculous idea, even by this day's standards. There was only one way he was ever going to find out anything, and John was running on very little patience today.

"Goddammit, whoever-you-are"—slap—"WAKE UP!"

This time the white-haired man jumped a mile high with a startled grunt, then looked up accusingly at John. "What the hell was that for? —Oh. Oh, that's odd." He was feeling his face: his relative lack of cheekbones, his jaw, his teeth. Suddenly he jumped up, forcing John to take a step back. "Not the least bit important, but still, odd, don't you think?" He grinned, eyes piercing.

John, pulling himself together, met the unfamiliar eyes warily, his planned vituperation not forgotten but perhaps better saved until he knew what he was facing. He could handle Sherlock, and he could handle a stranger, but he'd never imagined a man who was both. What had he unleashed?

"Not sure about this voice, though, really," Sherlock mused. "I quite liked it before."

John noticed Mrs. Hudson quietly leaving just then, and failed to catch her eye. Part of him wanted to scream. Am I crazy? Why am I the only one freaking out about this?

Because, John. Moirallegiance is a powerful thing.