Sherlock is back, then. John has found the flat at Baker Street dark and empty on his last few visits, and has only stayed a few minutes each time. The flat without Sherlock is sad, and even more so now that John is barely there himself.

But tonight the lights are on and as John lets himself in he can hear Sherlock in the living room, declaiming, and he runs up the stairs.

There are hearty greetings, exchanges of news, comparisons of the weather in March in England and Germany. Sherlock looks tired but bright, sheer nervous energy creating the glow. There are many coffee cups abandoned around the place. John clears a few up and puts the telly on and settles in his chair so he can relax and let Sherlock carry on working his way through a stack of Der Spiegel.

"Case?" asks John, eyeing the papers. He knows Sherlock has been doing more international ones lately. Rather sweetly he still offers to split the fee with John, even if John's sole contribution is to respond frivolously to Sherlock's terse keeping-in-touch texts.

"Practice," says Sherlock. He gives a mouth shrug, and glances at John with the slight smile which characterises their acknowledged relationship: steady loyalty from John, answered with amused tolerance from Sherlock.

John rolls his eyes and goes to make the drinks. He will stay here tonight, he thinks. It is too much of a slog back to Mary's. She will understand, she always does, especially when it comes to Sherlock. He'll text her later. But just now, he is enjoying being here, feeling like old times, watching Sherlock do what Sherlock does.

He is going to miss this.

He waits a long time before deciding there is no right moment.

"Sherlock, there's something I want to tell you. Mary and I are no longer going out."

Sherlock widens his eyes, takes in John's earnest face.

Mary. The latest in John's intermittent forays into female companionship. Dark hair, sparkling dark eyes, short and thin - elfin John called her - and jittery, understandable given the case she came to Sherlock with, a nasty ex boyfriend who she wish to prove was stalking her. So flaky was she that John ended up doing most of the interview because Sherlock had no patience for all the stammering. They solved the case and now the ex is being charged with stalking and threatening behaviour and will possibly end up behind bars. Then Mary still kept appearing at the flat and it transpired that John kissed Mary, rather a lot, in taxis travelling between locations, and then they were going out, which translates roughly as staying in at Baker Street or at Mary's Islington flat, having sex, and John disappeared from his and Sherlock's flat for days at a time.

Sherlock was glad to go to Germany, even as a favour to Mycroft, because John and Mary were euphemistically going out rather noisily in John's room one evening and Sherlock could not concentrate and had to leave the flat and walk around until he estimated that even they had to have stopped for rest.

In Germany he further calculated that the relationship would be finished by the time he returned. The constant sex stage would be over and the talking about the medium term future stage would have begun and John is no good at discussing the future beyond the end of any given week, which women find frustrating, and a sign of lack of commitment. They do not see, and Sherlock is not about to point out, that John committed to Sherlock within twenty four hours of meeting him, and has hardly left his side for a day since then. John can commit. But he cannot discuss it with his girlfriends whilst he is doing it.

Sherlock has also realised that Mary's case, which was not interesting or difficult, was brought to him because of him: Sherlock. She spent the interview, when not sobbing or stuttering, looking at him. His legs, his hands, his eyes, his mouth. His groin. He got up and turned his back and stood looking out of the window, feeling John's disapproval of his rudeness and not caring because he wanted to tell her he would not take the case but honestly it was quicker to solve it and send her on her way - minus the fee - than to try to justify a decision to someone with a clear obsession. He was surprised when she switched her unsubtle attentions to John, but not sufficiently concerned to warn John about her fickleness. The most likely scenario was that she would soon meet a new person she found interesting, and John would be cast aside. Until then, they are having a lot of repetitive-sounding sex, or as they would phrase it, going out.

And now, it seems, they are not.

He leans forward, studies John's face. Is unsure whether to offer commiseration or congratulation.

"We're engaged," says John with a flourish, and Sherlock has fallen for the oldest trick in the engagement announcement book.

He stares at John. "Engaged." The cosy clutter of their home - books, microscope, newspapers, crumpled curtains, his music stand and the layer of crockery on every surface - unfocuses and only John's face, beaming, remains sharp and clear. Sound becomes muffled and all he can hear is his own heartbeat, and that is distant too.

"We're getting married. In two months. May."

Sherlock gets up quickly. "John. Congratulations," he says. "Champagne, we have some, I'll get champagne."

He stumbles to the kitchen and flings back the fridge door. Stands there with his head in the chill air for a bit, vaguely rummaging. "Got it," he calls, not looking back over his shoulder. "Hold on, I'll go and get the corkscrew, think it's in my room."

He makes it to his room and closes the door behind him.

He sits down, stands up, blinks away tears. He hopes it is a few minutes before John realises you don't open champagne with a corkscrew.

He wipes his eyes on his sleeve. He is fine. It was just a shock. John, gone forever.

John opens the door. Sherlock stands perfectly composed, apparently searching for the corkscrew.

John gazes at him for a long moment, the way John does. Sherlock is not the only one who can read people. He knows John is very good at reading him. "You daft beggar," John says, smiling. "You're not supposed to cry til the wedding itself."

"Not crying," says Sherlock. "Looking for something."

John stops smiling. "Come on, never mind champagne, we'll crack a couple of beers," he says. "There'll be plenty of champagne in a few weeks."

"You get them open," Sherlock says. "I'll be there in a minute."

"OK." John goes off and Sherlock stands and stares hard at the periodic table with his hands pressed together, and his matched index fingers against his mouth. The elements swim and quiver before his eyes and not one of them seems stable or certain.

When even the steadiest element betrays you, what is left?