Sherlock allows John and Sarah to think they were alone the night they saw the musical.
He was there, of course, watching from a safe distance. He remembers how his chest constricted seeing the two of them wrapped in each other's arms on the couch, kissing and touching as lovers are wont to do, but it is nothing, nothing, compared to the bottomless pit of darkness into which he is flung when he sees John on his knees in front of Moriarty.
For three years he sits and waits and watches. Three years of cocaine and morphine and absinth and heroin, but nothing can make him unsee what he witnesses through the open window, or temper the rage that wells up inside him.
He wants not to care. He wants not to feel. He wants to run away. He wants to burn London to the ground and bathe in its ashes, stab his arms with a thousand needles, fling himself from the highest mountain into the lowest ravine, anything to make the unbearable ache in his heart stop.
He knows now why that posh little Vicomte looked so devastated sitting in that opera box, watching the one he loved being wrapped in another man's arms. The effect had been lost on him before, but it stings like a bullwhip now.
But he also knows sentiment will not save John, only he can do that, and that thought makes him strong enough to keep from coming completely undone.
The plan goes off without a hitch. More or less. It's something of a two man Mexican Standoff; Sherlock points his gun at Moriarty who in turn has two guns, one pointed at Sherlock and the other at John.
There's no dungeon, no dramatic posturing, and certainly no singing, but the situation is every bit as serious.
There's even an ultimatum to go along with it all.
It's unspoken, but it hangs in the air like a noxious gas. He watches the silent communique between John and Moriarty and he knows what's coming.
Sherlock is ashamed to admit it, but he has a contingency plan in place if he leaves this building alone. One needle, in his back pocket, containing the precise amount needed to cause a fatal overdose. The weight of it triples when John's looks to him for help.
Because he sees in his friends eyes that John knows. Not by any sign or signal, but because John knows Sherlock as simply and profoundly as his own heartbeat.
Together, they reach the same devastating conclusion.
Sherlock Holmes will be dead by morning, whether by Moriarty's hand or his own.
'Either way you choose, you can not win.'
And yet there is one thing no one took into account. One thing no one even thought twice about or cared to remember that would alter and define their lives in ways they could not have imagined.
She arrives, a wild, self-righteous tornado of feminine fury, and plants a bullet in Jim Moriarty.
Sherlock watches him writhe on the ground, makes a note to be kinder to Molly in the future, and erects an entire wing in his Mind Palace in her honor. It is a grand place, complete with walls painted in her favorite shade of yellow and an infinite collection of romance novels.
He considers tearing it down when she steps aside and allows Sebastian Moran into the room, but his mind grinds to a halt a moment later and he can no longer make rational decisions.
It doesn't make any sense, but unless his eyes deceive him, Moran reaches inside his leather jacket, removes his revolver, and empties it into Moriarty's chest. The look of surprise on his face is astounding.
Sherlock treasures every second of it.
Or he tries to, but even he finds it hard to express joy when someone you consider a friend points a gun at you.
He looks into Molly's face, stunned, as his mind races in circles trying to figure out what the hell me missed. It's something to do with Moran. It has to be. The Colonel is standing too close to her and is resting a hand on her arm.
She glances at him, then back to Sherlock. "No more," she says. "No more."
He nods, understanding her perfectly.
She lowers the gun.
Together, she and Moran leave. The door clicks softly behind them.
Weeks later, Lestrade will show up asking questions. Molly Hooper has quit her job and disappeared off the face of the earth and does Sherlock know anything about it? He won't, of course, because right now he's not thinking about the future. He's wrapping his trembling friend in his coat, pushing him out the door, and into a cab.
Inside 221B he strips John down and catalogues the abrasions scattered across his skin, every bruise and bite mark, until he knows the shape and size of them all by heart. He then pulls John into the bathroom, determined to scrub his body until every trace of Jim Moriarty is gone forever.
They wind up in the shower together, skin on skin, holding on to each other for dear life as the scalding water rains down on them. Sherlock wraps his arms and legs around John and presses him tight to his chest, burying his nose is short honey blond hair. John's fingers leave bruises on his waist, but he doesn't care. His mind is filled with 'John' and 'safe' and 'home.'
They stay in the shower long after the water runs cold and only leave once John begins to shake again.
He pulls back the sheets on John's bed and lays him down. The shock has not completely worn off yet and he moves pliantly when Sherlock tucks his legs under the blanket.
He straightens and is preparing to leave the room when John snakes a hand out and catches his wrist. "Don't go," he says. "Don't leave me again."
There's a lump in his throat. "John…"
"Please. Please. Sherlock. Don't leave me."
His eyes burn with something he decides are not tears and he touches John's face with his fingertips. "Alright."
He climbs into the bed and wraps himself once more around the smaller man, tucking his head beneath his chin.
The trembling gets worse and fingers grasp desperately at his flannel shirt, but he holds fast, waiting for the storm to break.
When the tears come, when the last three years finally catch up with John, it is as the many cracks in a dam all giving at once and the mighty waters rushing forward. The sounds coming out of John's mouth are not quite human, caught somewhere between sobs and sighs of relief they sound to Sherlock like the laughter of God.
"Sherlock…Sherlock…please," the fingers get tighter. "Oh, Sherlock, don't leave me. Don't go. I love you. Don't leave me again. Don't leave me. I love you…"
Between kisses and touches he listens to John's mantra for hours and his soul takes flight.
"I love you, I love you, I love you…"
There's no drug in the world that could take you higher.