Run after him.
That's Sherlock's first thought. But the second is Leave him alone, and he doesn't know which one is right, so he stands paralyzed in place until it's too late anyway, and John is too far to bring back.
The moment it's too late, Sherlock realizes he should have run.
He can tell by the silence in the flat, the way it presses down into his chest and makes it hard for him to steady himself, reaching out a hand to close on (John) the newel post at the top of the stairwell. This isn't right; there shouldn't be this mute testament to Sherlock's inability to help; there should be John, and Sherlock's arms around him, and quiet whispers, John, he was a liar, you are good, so good, and strong and brave and beautiful, and how can you not know?
But there is none of that, and he is sitting in John's chair, surrounded by the scent of him as he tries to think of how to fix something that is this broken.
His thoughts are wild, all over the map, from making tea for John (because, somehow, that always makes things right when it is John who makes the tea) to having his father executed (Mycroft can do it; Sherlock is sure of that), but he doesn't see how the first idea will help, and he thinks that the second one might be a Bit Not Good. So he sits, as the light fades into evening through the windows, and does nothing and hates himself for it.
The phone in his jacket pocket buzzes – adrenaline jackknifes through his chest – but the number is Lestrade's, and Sherlock remembers the case that started all of this, and that there are probably statements to be given and apologies to be made. He ignores the text, but turns the mobile over and over in his hands.
John, he types out, but deletes it.
Come home, but that feels stupid and meaningless and impossible, and he deletes that, too.
He is good at so many things; he knows it's true because John tells him every day, at every case and every crime scene (fantastic, brilliant, amazing), but at this, at anything that really matters, he is useless.
John also tells him that, sometimes, in kinder words, but he means buying milk and cleaning dishes and remembering that other people sleep at night. Not this.
It occurs to him that Mycroft will know where John is, and he wonders again if he should go to find him and bring him back. But if Sherlock was useless while John was still here, and useless now that he is gone, then what good will it do to chase him down and then stare, frozen, while he lets John slip away again?
John's words come back to him, and his hands twitch, tense into involuntary fists, white-knuckled at the way the sound carves out a hollow somewhere inside him. My father… I never wanted you to have to know.
Sherlock has his mobile open again, typing rapidly, before he is even fully aware of what he's doing. His fingers almost hesitate on the "send" button; doing this goes against all the aloofness and distaste he usually works so hard to maintain, but there is nothing usual about this, and it's for John. Or maybe it's for them both, because Sherlock's need to do this is stronger than anything he has ever felt (until he remembers a corridor and a breathless kiss and John and thinks, stronger than almost anything he has ever felt).
He isn't expecting a text message in reply, and true to form, his phone rings. Not the usual ring, but the one reserved for people whose calls Sherlock generally doesn't bother to take. There is a pang of guilt at this, and Sherlock resolves to take Mycroft off that list, because Mycroft would never stand by and allow Sherlock to be hurt – and he is extending that to John now, and that alone is enough for Sherlock to move Mycroft firmly to the group of people he tolerates, if grudgingly.
Mycroft gives Sherlock the information he has asked for, offers him a car (Sherlock declines) and, before he hangs up, stops to ask, "Have you thought about this carefully, Sherlock?"
"I don't need to."
"If not for yourself, for John."
"You didn't have to help me."
"No."
"But you did."
"Yes."
"Then I need no further reinforcement."
"Be careful, Sherlock. Think of John," is the last thing Mycroft says before breaking the connection.
The cab takes forty minutes to get there, seven minutes longer than the fastest route, but Sherlock doesn't care and throws too many bills into the passenger seat anyway. The street is quiet when he exits, filled with trees and too-tight houses crammed crookedly together.
It fits uncomfortably around Sherlock, too peaceful a neighbourhood to be familiar from case work, but too small and ordinary to remind him of growing up. He tries to picture John here, younger, but the thought only makes him angrier and he doesn't like it anyway, because John belongs at Baker Street and nothing else makes sense, especially not this mindless calm.
He finds the door with the right number on it, Mycroft's cool voice reciting the address in his head. Weathered paint over the wood – too much like Baker Street – a bedraggled garden, an old-growth tree shading the walk, all details that don't matter, but Sherlock has had over thirty years to learn to stop observing and he hasn't managed yet. There is a doorbell, but he knocks.
Briefly, nothing stirs. He hesitates to breathe, thinking irrationally that it might give him away; then there are footsteps, a voice from within the house, and a warm patch of yellow light on the step where he stands as the door is opened and a bemused face looks out.
"Mrs. Watson. Sherlock Holmes." A hand, offered and then withdrawn before she can move to shake it. Sherlock barely stops his lip from curling in disgust, but she isn't his reason for coming. "Is your husband at home?"
"I – yes, he is," she says, clearly uncertain about this tall, dark, forceful stranger glowering on her stoop. "Henry!"
Sherlock enters the house, brushing past Mrs. Watson in a way that brooks no argument, as the older man emerges from a back room.
"Hello?" It's plain he has no idea who Sherlock is. Good; Sherlock can introduce himself.
He's hit people before, in the rush of altercations on the job, and he knows how to do it. The training leaves his mind now, though, no thoughts of weak spots or stance or proper form, and his blind fist connects and cracks and suddenly the older Watson is on the floor, both hands covering his face. Sherlock can see blood, and of course it's just a nosebleed, but he wishes for a moment it were more, because this is nothing – nothing – to what this man has done to John.
Sherlock's foot against his chest stops John's father from rising.
"What the hell – "
"Mr. Watson. You and I need to have a talk."
The man on the floor makes a stifled sound, the swelling already rising to obscure the words he can't quite force out through surprise and pain and mounting anger.
"We have a mutual acquaintance, Mr. Watson. His name is John."
A muffled oath, cut off with a gasp as Sherlock bears down.
"Listen to me, because this is quite possibly the most important thing you will ever hear."
The pressure lifts, just slightly, enough so that John's father can suck in a groaning breath.
"John Watson is the best man I have ever known. And I've seen what you did to him."
The foot digs in again, involuntarily, as Sherlock remembers his soft touch on John's skin.
"And I want you to remember this, Mr. Watson. He is what he is in spite of you. In spite of torture, in spite of terror, in spite of your worthless – " dig " – opinion – " dig " – of your own son's value – " dig " – and you had better hope, with everything you are, that I never find out there is more to the story than I already know. Because if I do, Mr. Watson, if I do," and here he has to breathe, deeply; he is forgetting himself, and that won't do, this is about John, not him, "then no force on this Earth will stop me from seeing to it that you feel what John felt, every lash and every stroke, ten times over. And don't think I couldn't do it."
He lifts his foot away from John's father's shirt front, pausing to wipe it on the carpet – unnecessary, but he can't help but feel that everything that touches the man is somehow tainted – and turns his back, coat flaring out as he strides back toward the doorway.
He stops, framed against the quiet night, to say, "Remember this, Mr. Watson. Your son is better than you will ever be, and he is protected, and he is loved."