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"What the hell is that?" John asks when he reaches the top of the stairs. He stops in the doorway and tells himself that he's not actually seeing what he's seeing. Possibly Sherlock slipped him a hallucinogenic before he left that morning. It wouldn't be the first time, and possibly they should have words about that.
Sherlock shifts the small bundle of blankets in his arms, and it gurgles.
Dear god, it actually is--
"You're a medical man, John. Surely you recognize a baby when you see one."
He does. Roughly three months old, blue eyes staring up at Sherlock, watching him, one hand batting at his chin.
"Yes, but what is it doing here?"
"Someone left it on the doorstep," Sherlock says with a nod to where John's standing.
"No," John says, finally daring to come the rest of the way into the flat. "Someone absolutely did not because that sort of things only happens in films."
Sherlock frowns down at the baby. "Does it? Interesting."
"Where did you get it? Did you steal it?"
"Why would I steal a baby?" Sherlock asks, eyebrows high on his forehead, and it's hardly as reassuring as it should be that Sherlock seems to find the idea ludicrous.
"I don't know! Why did you raise koi in the bath for three weeks? Why do you leave heads in the fridge?"
"That is a perfectly logical place to keep a head. In fact, the only logical place in the flat. Next time, I'll leave it in your bed, shall I?"
John shuts his eyes and takes a deep breath. "Sherlock. The baby."
"Doorstep. Pay attention."
"Right. We live in a film now. I forgot. Was there a note or a... something? Anything?"
"Don't need a note."
"Of course, you don't." John sits down on the sofa and gives Sherlock a wave. "Off you go then."
"Blanket's expensive. Could be stolen, but more likely bought. About as hold as he is, laundered several times, but in good shape. He's in good health, good weight, clean. He's clearly been looked after. Had a bath just this morning. So unlikely he's been abandoned for monetary reasons."
"Why else does someone give up a baby unless they can't support it? And he's been left with you. Not with family or the hospital or the police. What does that mean?"
"Still working that out."
"It's not... I mean, could he be, well..."
Sherlock stares at him blankly, waiting for him to finish.
"Yours?" John concludes weakly, the idea of it practically too absurd for breath and yet, at this point in their relationship, nothing Sherlock says or does ever really surprises him. So it's possible.
Or not, given the withering look Sherlock is now shooting in his direction.
"Sorry, but if we're using films as the basis for reality now, I have the right to assume that the film version of you has fathered dozens of illegitimate children the world over."
Sherlock smirks briefly, but then the baby starts to fuss. John watches as Sherlock shushes at him, shifting him to his shoulder, and slowly pacing the room. It isn't long before the baby settles, eyes drifting shut, tiny hands curling and relaxing against Sherlock's arm.
"Why are you so good at this?" John asks.
"I have several small cousins. I looked after them a lot when I was younger."
"You?"
"I liked them because they were too young to be idiots. They liked me because I could figure out what they wanted."
"You can't solve a baby, Sherlock."
Sherlock glances down at the baby now asleep contentedly on his shoulder, then back up at John. "Can't you?"
John considers arguing for a moment, but Sherlock's looking entirely too pleased with himself, and besides, the baby's just fallen asleep.
"You've gone all broody," John says with a grin. "I was only gone an hour, and now look at you."
"No, I haven't!" Sherlock makes a good show of looking scandalized by the very idea of it, but John just grins wider, chuckling quietly.
"Have you phoned the police yet?" John knows he hasn't, knows he hasn't put that baby down since he first picked it up. "Because you can't just keep a baby that shows up on your doorstep."
Sherlock walks the length of the room a couple more times, rubbing the baby's back, before he finally sighs. "All right, fine."
"You sure?" John asks, still laughing, as he gets to his feet and pulls out his phone.
"Oh, shut up," Sherlock says, but it's lacking its usual venom.
John gives Sherlock's shoulder a squeeze and dials Lestrade.