A/N So this one was inspired by one, the genuine belief of opening alternate worlds with every decision, and two, The Subtle Knife by Phillip Pullman, from His Dark Materials series. I've always loved that idea - the birthing of all sorts of different worlds with every pivotal choice, worlds where things could have gone so differently. I love thinking in what-ifs and might-have-beens, and so this fic was born.

Thanks very much to quantumspork (on Ao3) for being a patient beta; you gave the best suggestions and encouragement. Un-Britpicked thus far; viewable on Ao3 under the same username.

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It begins like this:

John Watson, taking a walk in the park, gait leaning heavy on the cane and brow furrowed at the world. Mike Stamford, taking his lunch on a park bench, because he's just had a trying day of lab practicals and needs the room to breathe. John limps past, Mike looks up, and there they are. Poised to break off, be born: all the alternate universes resting on this choice.

In one of them, John does not look back. He strides off, ashamed at having been recognized, too bitter with his lot to want to see a familiar face. Mike Stamford sits back down, wondering if perhaps he'd been mistaken. Sherlock Holmes lives alone.

In another, John turns, if only to say hello. Mike brings up the army and John grows quiet. The conversation peters out and John finishes his coffee. Sherlock takes Lestrade's case and arrests the cabbie right at his doorstep.

In yet another, John turns and they talk. Mike asks about living arrangements and John laughs. The offer to meet Sherlock stands but John refuses. One day later, Sherlock matches wits with the cabbie in an empty library at a small college. He loses.

In this one, however, John Watson turns, and Mike Stamford laughs, and they take coffee on the bench. They end up in the Bart's lab, where a strange but brilliant man hovers over a microscope and lies about the signal on his phone. And as Sherlock Holmes sweeps out with a cheeky wink and a dramatic swish of coat, John is left with this strange feeling that this may be one of the most important points in his life thus far and ever, as well as one of the strangest and most terrifying ones.

He takes his time heading back to his bedsit because there isn't much to pack up, anyway.

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Does it really begin this way, though? Or does it begin before, when John decides to take a walk because he doesn't want to get home – a word used loosely; the bedsit feels too transient to be called "home" – too soon? Or before that, even, when John is on his knees under the Afghan sun with his fingers plugged into a young man's artery, and feels the split-second numbness of tearing skin as the bullet rips through his shoulder? Or before that, when John signs up for the war? Chooses to take his placement in the army instead of a normal hospital? Applies for a medical degree?

Or does it begin even longer ago, tracing down all the tangents in time, back and back and back, running through all the points at which things could have gone differently? All the little points in history that, had anything been different, Sherlock Holmes and John Watson would not meet? Countless parallel universes, then, wherein they never would have – where John dies in Afghanistan from a gunshot or Sherlock in London from a knife; where John chooses to work in Royal London Hospital instead of the RAMC or Sherlock becomes a criminal, too sick of pandering to the police; where John fails out of medical school or Sherlock sinks so low into drugs he cannot hope for release. An inordinate amount of what-ifs and might-have-beens, but the two of them find themselves in this reality, in this here and now. And here and now is John sitting at his desk, fingers poised over his laptop keys, one name running in his mind. Here and now is Sherlock surveying his tiny flat on Montague Street and wondering how he'll manage to move everything over. Here and now finds the two of them teetering on a single decision, and John Watson will never know just how precarious that balance is as he hits the search button to look up Sherlock Holmes.

He packs but doesn't have anything shipped. He's not quite decided just yet.

All the other worlds lie in waiting.

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Perhaps: the next day, Sherlock leaves the flat and suffers Anderson's idiocy for the forensic analysis. John takes a cup of tea with Mrs. Hudson over a few shows on the telly. Sherlock comes home with the pink case, and John shakes his head and eats a biscuit. Sherlock goes to Angelo's alone.

John limps the rest of his days.

Or maybe: Sherlock offers and John refuses; he stands firm – too much, far too much for a lifetime. He gets work at a local clinic the next day and Sherlock tiptoes his work around him. John eventually marries Sarah and moves out.

Sherlock lives on alone.

In another world, Mycroft Holmes finds the barb that ensures John Watson will never go near his little brother again. And in another John takes Mycroft's offer. In another John doesn't get up to follow Sherlock to chase a cab; in another John balks at the jump on the roof.

In this one, John runs through London's back alleys and side streets, and they accost a poor American tourist who is not their killer. They arrive back at 221B breathless, giggling, the thrum of adrenalin hot in their veins. John looks at Sherlock and Sherlock looks at John, and there is the slightest of hesitations.

A different John Watson would kiss him. That John Watson would pin him to the wall, wind fingers tight through those ridiculous curls, crush their lips together. That John Watson would blow him right there and then on the stairs, until Sherlock Holmes was a shuddering, shaking mess.

This John Watson shakes his head as Sherlock cements his place in the little flat.

The cane, he has to admit, is a nicely dramatic touch.

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The pill gleams in the light, little speckles stark contrast to its smooth, white sheen. The cabbie stands, waiting. Sherlock feels the tiniest frisson of doubt skitter down his spine – is it, is it, is it, yes, maybe, no, of course, but if–? Two pills, two choices; a fifty-fifty chance, just as he'd said.

It's only luck.

His hand shakes as he slowly brings the pill to his lips.

He swallows. He lives. Or: he's wrong, he dies. Or: he throws the pill aside and leaves.

Reality: a shot rings out, smashes the glass, catches the cabbie not two inches up and to the left of the heart. Instantly fatal. He pulls a name from the dying man's lips, a unique and wonderful name: Moriarty. As Sherlock is pulled away from the body and draped in an absurd, orange blanket (it does say plenty about the deplorable state of the average human mind, how it can apparently be assuaged by a spread of luridly colored fabric), he catches sight of the other pill bottle still on the table and wonders if he was right. He will never know, because all the other possibilities are closed to him now, and anyway he is thankful he hadn't found out, because if he had then he might not be standing here, looking down at John and thinking, you are proving to be interesting indeed.

He's wrong about one thing, though: it's not fifty-fifty. At least a dozen alternate realities have bloomed because of that one choice made by a former soldier. But Sherlock doesn't believe in alternate universes or idealistic convictions or nonsensical ideas. He does believe, however, as he looks at his new flatmate in the other armchair, laughing over his dim sum, that John Watson is worth letting into his life.

He looks at John and thinks, you are going to be bad for me, aren't you?

John sips his tea and crinkles his eyes at Sherlock in a warm, honest smile.

Sherlock predicts another fortune cookie, just to turn that smile into a laugh.

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And so it goes on. Day after day brings world after world into existence, a plethora of them hovering around, unseen and untouchable. In reality John asks Sarah out, while a different John simply smiles and goes home to where Sherlock waits naked in bed, and yet another John is completely indifferent to them both.

The pool is a feast of might-have-beens, because so much rests on one slender finger shaking around a trigger. Jim Moriarty smirks and watches, and John realizes then and there that indubitably, his trust wholeheartedly lies in the brilliant and terrified man before him, down to his life.

It frightens him, but it gives him something else too: he is a soldier now, again, and there is a life he must protect, a man he must save. He feels something click inside of him, something that settles, that grounds him to the here and now of everything.

And so: when Sherlock shoots, John tackles him into the pool, catches the brunt of the explosion. He is gravely injured, but they survive. Moriarty does not.

No: Sherlock shoots, John lunges, nothing catches in time. The explosion throws them backwards and John, having dealt with so many explosions before, cages Sherlock in his arms. Shrapnel lodges in his chest and Sherlock's first explosion bigger than a chemical imbalance kills a man right before his eyes. John pins him down with dead, dead, dead weight and the man's last efforts are to make sure Sherlock is all right. He dies with his fingers on Sherlock's pulse.

Moriarty disappears.

Again, no: Sherlock's finger shakes on the trigger. His eyes flick to John to the vest to Moriarty and back. John braces himself but the shot never comes. A phone rings out and things happen that he does not understand.

That is not to say he isn't relieved. Now that his life is irrevocably tied to Sherlock's, he doesn't want to find out what it's like to have that taken away.

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There are all the times they could have kissed, where they do in other worlds:

in a darkened tunnel after Sherlock saves him from Chinese smugglers, Sherlock desperate and John just barely keeping up;
at the hospital, in John's room, his injuries from struggling prior to kidnapping patched up, Sherlock apologetically wrecked and John relieved beyond any words;
on the night Irene dies and Christmas is strange, with John in his chair and Sherlock whirling into the room, an indomitable force that searches for answers in a forceful kiss, answers that John gives in every touch that sears Sherlock to his bones;
in a dark room in Dartmoor, after Sherlock wakes from a nightmare, each press of lips soft and languid as John lulls Sherlock back to sleep, and neither of them think anything of it anymore, because kisses are just one more thing between them;
in a lab at Bart's, one soft press of Sherlock's lips to John's graying hair, because he knows what comes next and knows there will never be another one after – their last kiss, and John not even awake to feel it.

There are times they could have died, and these they both can imagine very well, because these keep them up nights (will it surprise them so, to find out how hell bent they are on keeping each other alive and intact, because to lose one is to lose everything?) and these come so often they are not allowed to forget:

John with his throat slit in Baker Street, the dead man symbols scrawled on the windows in yellow spray paint, Sherlock just seconds too late;
Sherlock with a bullet thudding through his brain and John falling to his knees, horror an understatement for the clawing feeling in his chest, because how could he have missed, how could he have missed?;
Sherlock, careless and frightened and shaken, coming to a halt on the moor because of something shifting under his foot, buried in the ground, and John's utterly wrecked expression because why here and why now and Jesus, no, why him, please not him and too late, soldier, there are things you cannot save people from.

The possibilities keep them looking at each other, at odd times and in odd, intense ways, just because they liked to be reassured that they are both still around with beating hearts.

Everyone always assumes in the relationship, John is the heart. But if one looks into Sherlock's eyes, as he watches John when he knows John is not looking, one would assume something very different indeed.

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If John had known about the parallel worlds, perhaps he might have decided things differently today. Perhaps, if he had known, he might not have left Bart's. Or might not have let Sherlock go off on his own, on that empty street outside Kitty Riley's flat. Might have pleaded Sherlock's case to Lestrade. Might have done other things, prevented this somehow, this now-inevitable destruction of the best man he's ever known. Because now finds Sherlock standing on the ledge of St. Bart's roof, phone held to his ear, arm outstretched toward John as if trying to breach the impossible gap between them. Now finds him looking down at John, who seems so impossibly small, which isn't right – John is so big he takes up all the space in a room like light. Now finds John reaching up to Sherlock, pressing the phone to his ear as if trying to push into it somehow. Sherlock tells John to stay where you are and John is so desperately confused and simply desperate, and –

– and in another universe Sherlock is kissing John up against the post of his door frame, with John's hands tangled in his hair and Sherlock tugging up that deceitful jumper, and –

– and Sherlock tells John please could you do this for me and John asks do what? and –

– and Sherlock is mouthing down John's spine, pinning the man to the bed with his weight as he reaches around, does things with his fingers that shake John to pieces –

– and Sherlock chokes it out, that this phone call, it's – it's my note because that's what people do, don't they? Leave a note and John, oh John, does not want to understand; just this once, he doesn't want to understand what Sherlock means, absolutely refuses to want Sherlock to be right –

– and Sherlock is curled around John, tangled up in him and the sheets, his front to John's back, tracing S-H-E-R-L-O-C-K over the scar again and again. John dozes to the repetitive motions and only then does Sherlock lean forward to mouth "I love you" over his pulse point, the direct connection to his heart –

– and Sherlock whispers goodbye, tosses the phone away, and looks down, heart weighted by the sheer number of things he wishes he might have said. In both universes John shouts Sherlock's name, but one is in ecstasy and the other is in sheer anguish. Like a bird or a falling angel Sherlock plummets, arms pinwheeling, hair flying everywhere.

Every parallel universe they've ever created, every tangent that has gone off from their choices, shudders with the force of a body hitting the pavement.

In another world, they might have been. In many other worlds, in fact, they are – they are Sherlock&John, two inseparable forces entwined by a singular and unique twist of fate. The lonely consulting detective and the washed-up army doctor; who would have thought they went so perfectly together? And yet they do – in so many, wonderful ways they do. They by no means complete each other, for that idea is absurd in its own right, but they complement in ways that make each other shine the brighter, and their contradictions line up in ways that make them work.

In another world, they grow old together, Sherlock and John. In one of them, one of the best possible alternatives, Sherlock kisses John among the bees, autumn raining leaves all around them. The country house is quiet in the evenings, and Sherlock is cranky about haggling over the honey, and John is quite good friends with the owner of the local pub. Lestrade drops by occasionally for a little chat and a few beers. Mycroft now rules heaven, or so Sherlock likes to think. (Actually, he believes Mycroft rules hell and forces everyone to make him little cakes, but he can't tell John that or John will mess up his sock index, again).

In another one of the bests, John sits by Sherlock's bedside in the hospital, one pale hand clasped in both of his worn ones. They look at each other and give sad smiles that say worlds to each other. John kisses the backs of Sherlock's knuckles, and Sherlock settles in and begins his usual recital of everything John had done that day until he is too tired to be stubborn about keeping on speaking. John watches the heart monitor as it beeps slowly down, so slowly, until it is nothing but a straight line.

According to Sherlock's beloved mathematical principles, the shortest distance between two points is a straight line. John looks at where Sherlock's heart is no longer beating, and at where his steadily beats on, and thinks of the distance now between those two points. He doubts even his mad, brilliant genius could have solved for how to graph that space, two feet in a hospital room but infinity everywhere else.

But in the only world they know, John tumbles onto the pavement in a daze, unable to see the last of Sherlock's fall and hating it. Other people reach Sherlock before he does, other people check him over; other people are taking the last of Sherlock away from him. All John gets is a touch of wrist and a sight of blood-matted curls.

All John gets is to call Sherlock his friend.

All Sherlock gets is to hear it.

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Sherlock doesn't believe in alternate universes or idealistic convictions or nonsensical ideas, no, but he does wish, as he watches John limp away from his grave – a very different limp from the one Sherlock had cured him of, for very different reasons – that things could have been any other way. He would give anything to be in a different world right now, a world where he might still be alive and whole in John Watson's eyes. A world where he has not just ended that of his friend's.

To walk away from John is one of the hardest decisions he's ever made.

But the world that he makes, the alternative that he can live in and touch and experience, is worth it. When he comes home to Baker Street one cold, lonely night, worn and weary, having lost so much but kept safe what matters, it will be the best possible world there is.