The first time Sam and Dean Winchester meet Sherlock Holmes, they're hunting down a werewolf and he's in the way.
Sherlock Holmes is thirty years old, living alone in a flat near Charing Cross Station. He works as a consulting detective for Scotland Yard, always alone, and always unpaid. When he gets the call for an unusual victim on Hackney Road, he picks up a trench coat, ties a pale gray scarf around his neck, and runs out the door.
It's a case, his first case in eight days, and he's been bored out of his mind. There's no one to talk to but the skull on the bookshelf, and Lestrade is boring.
Still, something piques his interest about this case. But it's not the body that interests him.
It's the two men pretending to be M16 agents on the fringes of the crime scene.
He ignores Lestrade's attempt to apprise him of the situation, instead pacing over to the two curious men. They're talking in hushed tones when he approaches.
"Sherlock Holmes, and you are?" He offers a hand, which the shorter one shakes. Shorter being a relative term, since the man is precisely one inch taller than Sherlock himself.
"Agents Tom and Phillips." He flashes an ID for a purposefully short amount of time, such that Sherlock cannot properly inspect the forgery.
"Real names, please." His eyes rake up and down the two men – no, brothers, enough of their genetics overlap – analyzing everything.
The shorter one's expression falters for a moment, but then the smile returns. "I'm not sure I understand you. We were sent here, we're M16."
"No, you are clearly not. Your accent is terrible, first of all, so you may as well drop it. Southern United States, am I correct? But trying to forget that part of your past. Both military men, clearly, but not secret agents. Which leads to the real question – why are you impersonating agents to get into this crime scene?"
The brothers exchange a worried look. Clearly the shorter one is the one giving the orders. He's older, too, judging by the way the taller one stands just behind his shoulder.
"Maybe we could discuss this somewhere more… private?" He looks around uneasily at the police officers.
Realistically, Sherlock should turn them in there and then. But he likes to keep an open mind.
His mind is otherwise occupied anyway, trying to identify the mystery men. He runs through every known list of American fugitives from the South, until he finally latches onto a name, an image. Oh.
His face must reflect his surprise, because again they exchange an uncomfortable look.
"Sam and Dean Winchester. A bit far from home, aren't we?"
He leads them out of the building. A small part of his mind warns him against isolation with registered serial killers, but Sherlock relishes the danger. He's been bored for a week, after all. This is the most fun he's seen in months.
"How did you-" At least Sam's dropped the accent.
"It's obvious. Your suits, while expensive, are not worn every day. The clear creases show they're stored for long periods of time between wears. Not agents, then, because you don't do formal work every day. But you've clearly done this before; you're not amateurs. So, Americans with a reputation for impersonating federal agents. Clearly uncomfortable around actual officers, so probably fugitives. Only takes a few moments to match the names to the faces. But really, impersonating agents, do you think you could be any more conspicuous? Last I heard you had half the FBI on your tail."
Dean shakes his head. "That doesn't matter. There were tons of officers in the room; why didn't you alert one to our presence?"
Sam nudges his brother. "Dean, you know what they say about looking a gift horse in the mouth…"
"You intrigue me," says Sherlock. "I have, in fact, been following your case for some time now."
Dean rolls his eyes. "Lovely. We've got ourselves a fan."
"You are not serial killers, despite what the police are saying."
"There you go. At least one person believes us!" Dean says to his brother. Then, back to Sherlock, "But, uh, why? Frankly, the evidence against us is a little overwhelming."
"Perhaps. But I looked further than the police reports. In all the places you've been spotted, there have been mysterious murders directly before your arrival. Children disappearing, people turning on their loved ones, the like. And once you two arrive, the killings stop."
"Finally, some appreciation. If only Hendrickson could hear this guy."
"Special Agent Hendrickson and the other FBI agents do not see this pattern. They only see the string of casualties you leave behind."
"But you're ignoring that?"
"Like I said, casualties. You are doing good. I can only assume the two of you are in the same line of work as I am."
Sam steps forward. "Honestly, who the hell are you? You know a lot more about us than we do about you."
"Sherlock Holmes. Consulting detective for the Scotland Yard. I do believe we'll be working together, gentlemen."
They work out a kind of shaky alliance, wherein Dean and Sam tell Sherlock what he needs to know and nothing more. He knows that there are many things they will not tell him, or cannot tell him. But for once in his life, Sherlock does not pry. He doesn't want to be another one of the deaths that follow the Winchesters wherever they go.
They work together, outside the police, because if Lestrade learned that Sherlock was working with known serial killers, he'd never get a case again.
Still, with each minute Sherlock remains in their presence, he's convinced time and time again that they are not the villains. Evidence points to it, and Sherlock has always been one to trust evidence, but in their case there is much more than meets the eye. Dean and Sam Winchester keep many secrets, not just about this London murder but about their lives in general. Sherlock is missing something from this.
Sherlock learns quite a bit about them through observation, however. Dean watches his younger brother very protectively, like he's afraid Sam will break. Sam seems mostly innocent, but there's something dark underneath his puppy-dog demeanor, something Sherlock can't quite place. He hates it, not knowing everything. These men fascinate him. When the murder mystery is solved, he'd like to invite them for a cup of tea.
They tell him bits and pieces – this murderer is unlike anything he's ever seen before, or ever will see again, but he'd better not ask too many questions about it. Yes, it likes to hunt at night. Yes, they can stop it, but only with silver. Sherlock's never called a murderer an "it" before. Statistically, this murderer should be a "he." But it's the pronoun Sam and Dean use, and so he plays along. Sherlock is unused to taking backseat in an investigation. He doesn't like it.
The action culminates one night when Sam and Dean go out to stop the thing, and Sherlock's left behind in his flat. He hates missing out on the action, but they were quite insistent. Held him at gunpoint and everything. Normally, he would just follow the two anyway, but Sherlock gets the feeling that in these circumstances he's in over his head.
They return in the early hours of the morning, sporting several large gashes and bruises. But they tell Sherlock he won't have to worry about more mysterious murder victims.
Before the Winchester brothers return to their home country, Sam slips Sherlock a phone number. He locks it into his phone, then hides the slip of paper in the top shelf of his closet.
He tries to forget the case as soon as it's over, goes back to working with Lestrade. Murders that once seemed complex are now simple, compared to working with them. Working with those brothers, who clearly knew more about the world than he did, was terrifying. He felt lost, confused, as if missing something important, for the first time in his life. There are more secrets in this world than he'd originally imagined. Sherlock has never believed in unsolvable riddles before, but he thinks he's finally come face to face with one. The Winchester brothers are his unsolvable riddle.