They get married for a case.
Well of course they do, of course they get married for a case, and the only thing surprising about this is how long it took Sherlock to find a case for which to get them married. A little over three years is how long, three years since they met, but he's only been keeping an eye open for one for the last year. It's still surprising.
It's mostly an experiment (of course it's an experiment) to see whether John can even be persuaded anymore to have that imaginary sexuality crisis. A year ago Sherlock noticed—sloppy of him, it had been true for months—that not only had John long ceased to correct others 'couple' assumptions, he was long since utterly unbothered by them, had stopped searching for the moment it had happened. Thus the experimental marriage. If there is one boundary Sherlock hasn't broken that could finally set off alarms in John (besides the obvious and ugh, no, not even his first marriage to his work is that physical) this is it.
And yes, all right, the experiment is only partially about whether it's possible to run John off. It's also about whether John intends to run off, which is a different thing entirely. A normal person might, if they felt the need, say casually "You're welcome to never leave, ever, by the way, if you like. How does that strike you?" Sherlock Holmes starts stealing Lestrade's active case files looking for one he can use to get them married.
John is sitting on the couch reading the paper the morning after Sherlock finds the perfect case. Sherlock comes in, picks up John's cup of tea from the table as though John made it for him, ad plops down in one of the chairs in a wide-planted stance only describable as "battle ready."
"John, will you marry me?" he asks, as someone else not-Sherlock might inquire after the score for the match yesterday.
John flicks his eyes up to Sherlock, registers confusion, down to his tea, registers annoyance, back up at Sherlock, hesitates in consideration, and then looks back down at his paper.
"You're going to make me say yes," he says. It's not a question.
"You can say no."
John snorts, because well, that's a lie.
"Yeah, all right," he sighs after a long moment. "I'll have to get my good suit laundered. There's a stain on the front I've been meaning to see to."
"I'll send it out with mine," says Sherlock automatically.
John nods absently, frowning at a headline. "Thanks."
A pause. Sherlock sips John's tea.
"It's for a case," he says.
"I figured," says John. Another pause. He looks up from his paper again. "Did you want to tell me about it?"
"Only if you want to hear," says Sherlock mildly. And John snorts again because that's a lie too. But he smiles.
"After the paper, then." Sherlock nods but continues watching John. He hadn't actually expected the imaginary sexuality crisis, but he hadn't expected this either.
"We'd have it annulled immediately following the case," he clarifies. John's eyes are still on the paper.
"I should hope so," he says, but not in tones of any real concern. Without looking up he tugs out a section of the paper and hands it to Sherlock. "Here, go look at the obits. I'm reading."
Sherlock takes the obituaries page and finally goes, a broad smile on his face that John doesn't see. John looks up and calls "And leave my tea!" but a moment too late.
John goes to pick up some rings without Sherlock, who is hard at work on the case although he oddly doesn't seem that concerned about the outcome. He doesn't express any opinion on the matter of the rings, so John gets just a crackerjack pair, the cheapest the jeweler has. If they're expensive, he reasons, he'll feel like has to go to the bother of selling them after.
Before the day is even out and long after the morning post makes its usual rounds, a small package arrives, containing two ring boxes holding lovely heavy gold bands, each with a bit of tasteful decorative engraving, nothing too showy. There is a note enclosed.
Congratulations on this happy occasion, says the note. But let's try to make your devotion to each other at least a little bit convincing, shall we? – MH
Sherlock considers this to be an unnecessary abuse of both the postal service and coy irony.
Lestrade doesn't make jokes about the marriage like everyone else, but also doesn't really acknowledge John's insistence that it's "for the case." He seems to take it more seriously than almost anyone else, including Sherlock and John, and it makes John a little uncomfortable, to be perfectly honest.
The case is solved without incident, although it takes Sherlock much longer than things of the type usually do, and he doesn't brag about the conclusion. John only knows it's over because they go out for Chinese.
About a month and a half later, John is sitting in Gregory Lestrade's office wrapping up some paperwork or other that Sherlock couldn't be bothered to come in for. Lestrade is rifling for something in his desk and the paperwork reminds John of the forms they picked up the same day of their wedding before they left the courthouse.
"I don't even know where the annulment forms have got to in that disaster zone Sherlock calls a flat," he comments. He sees that look, that look that Lestrade gives him, but he firmly pretends not to. "I'll probably never find them. I ought to just go pick up new ones."
This time Lestrade actually sighs. "John…"
"What?"
"Nothing."
"Greg."
"Are you waiting for me to tell you that it's okay if you don't want an annulment?" he says. "Because I will. Because it is."
"What?"
Lestrade looks back down at his desk drawer with a resigned shrug of the eyebrows as though he wasn't really expecting John to take it well.
"Greg…"
"Forget I said anything."
"I'm straight."
"Of course you are."
"Greg!"
"No, I mean it," Lestrade insists, holding his hands up defensively. "Of course you are. I don't doubt it. And yet the fact remains that you're married to your flatmate—"
"For a case!"
"That's been over for a month! And you just sort of haven't bothered to get it annulled. You still wear your rings, for God's sake."
John crosses his arms over his chest, ring hand tucked out of sight. "What are you saying, then?"
The detective laughs, a little exasperated huff of breath. "God forbid I should dare to say anything," he says, shaking his head. "I just mean… Well, nothing about Sherlock's ever followed any rules. Maybe this doesn't either. Maybe for you blokes marriage just means taking each other off the rest of the world's hands." He sits back in his chair with a wide-flung gesture. "A publically declared unwillingness to move on."
John untucks his ring hand and stares down at the gold band, frowning mightily. Lestrade watches him, his own arms crossed now, eyebrows raised in challenge.
"We're just friends," John tries.
"Please," says Lestrade. "Friends isn't the word."
That evening, John gets home from some errands, comes straight into the kitchen where Sherlock is at his microscope, and holds out his hand.
"Ring," he says.
Sherlock has maybe a bit of a pang. Just a little one. Because he's perfectly fine with an annulment. Really he is, John's already proven all he needs to. But he'd stopped expecting it two weeks ago, and he's gotten to like the weight of the ring on his hand. He suspects his finger will feel a little naked without it. But the barely-there, nothing-at-all-really pang takes less than a second, and he pulls the ring off his finger and drops it in John's open hand. He turns back to his microscope quickly, and John busies himself at the sink.
Sherlock assumes he is making tea, but a few minutes later he realizes that the kettle hasn't whistled on time, and for that matter, the smell is distinctly un-tealike. He turns around, curious.
John is standing facing the sink, so Sherlock can't tell what he's holding. But the smell is coming from an open jar of jewelry polish on the counter next to him on the worktop. Sherlock stares at John's back until he turns around a dishrag and both their rings in his hand. He holds Sherlock's out to him, and Sherlock opens his hand dumbly.
"You can tell a lot about the state of a man's marriage from the shine on his wedding ring," says John with a wry grin, and drops the ring in his palm.
Sherlock's answering smile spreads across his face. It's wide and foolish but he can't help it. "Wouldn't want people to talk."
"People do little else," says John, replacing his ring. He screws the lid on the polish. "Tea?"
"Please," says Sherlock, and pushes the ring back onto his finger.