John hates east London at the best of times.

The cab driver had favoured him with an odd look as he dropped him off outside the warehouse, which is deserted and looks impenetrable.

"Wait here for me, okay? As long as it takes, put it on the meter."

And now he's stumbling around half-blind trying to find anything that looks like an entrance, and the crime scene tape rears up out of the darkness just as he's starting to feel desperate, guiding him.

He'd looked properly at his phone in the cab, the missed call list. Five from Sherlock, just after 2am. Two from Lestrade, an hour later. Then nothing, for hours. At 5.37am, 5.38am, 5.40am, 5.45am, Lestrade called again. There is another single missed call from Sherlock, at 4.57am.

Mycroft's time of death was estimated at 4.50am. It's that single call that haunts John the most.

It's even darker inside, and he walks gingerly through what looks like a vast, almost empty storage locker. He's half-afraid of disturbing evidence although he knows the scene has already been combed.

There's a bloodstain some five feet to the right of him, caught ominously in a narrow beam of moonlight, and he's starting to see now how the scene looked. The far wall, twelve bundles of rope lined up to represent where each hostage had stood, jagged red stains marking out Mycroft's position, the farthest on the right.

"Sherlock?" he says loudly, suddenly wanting nothing more than to find him. "Sherlock?"

He squints into the darkness, searching for corners, crannies, but now his eyes are adjusting he realises this is one large, smooth-edged space, and there is nowhere. Sherlock isn't here.

God, he isn't here.

Despair threatens to overcome him for a second. He keeps walking, heading for the opposite side of the room where there's a raised platform and a smaller doorway, with a horizontal metal bar. An exit.

He pushes through and emerges back into the night air, now in what looks like a disused car park.

And sees him from a distance, sitting on a packing crate with his face in shadow.

"Sherlock!" he shouts, sprinting flat out. "Sherlock!"

He doesn't look up, even when John reaches him and sinks down onto his knees, to eye level.

"Sherlock."

His eyes are glassy, fixed on mid-distance.

"God," John murmurs, taking in the blood on his shirt, splashes reaching up his neck. Mycroft's blood.

He takes hold of Sherlock's hands and he blinks, looking genuinely surprised.

"John."

"Hey."

"You're here."

"Yeah, of course I'm here. I'm here." He wants to keep repeating it, in penance for the absence he can never take back. "God, I'm sorry."

"Bristol, I assume?"

"What? Yeah. That's… yeah."

"You'd want to see Harry, of course. The case reminded you of her, and you haven't been in contact for just under seven months. Did you meet?"

Sherlock's speaking very quietly, very slowly, a voice that isn't his.

"Um, no. I don't even have her address. I just…it was pointless."

Silence, for a long time. He can't think what else to say, but keeps holding Sherlock's hands, and after a while Sherlock's lips begin moving, his eyes unfocused again.

"What?"

"If you throw me from the window, I will leave a grieving wife. Bring me back, but in the door, and you'll see someone giving life."

Nonplussed, John's about to ask how long it's been since he slept, but something Lestrade said is echoing in his head.

"That's the riddle, yeah?" he asks gently. "The last riddle Moran gave you? Probably won't come as a huge surprise to you that I'm stumped."

"It was too simple," Sherlock hisses, vicious suddenly, and tears his hands away from John's. "So obvious, I should have… yes. Window, widow, door, donor, too simple, too neat, not the kind of note you end on. Not the kind of note he would end on."

"I'm lost."

"The letter N, John, it's the letter N, forming the word 'widow' in the first instance and the word 'donor' in the second. Nursery stuff. I should have known instantly. Idiotic, even to go along with him, if I'd just…"

Sherlock trails off, eyes wild now.

"Sherlock, there's nothing you could have done. Christ, you solved all twelve riddles in as many minutes, there was literally no more you could possibly have done."

"Of course there was."

"Listen. I know I wasn't there. But from what Lestrade told me, and from what we know about Moran… this was only ever going to end one way. He was always going to kill Mycroft, from the moment he had him."

He winces at his own words; they come out harsher than he intends but he has to get this through to Sherlock, has to pull him out of this particular self-destructive thought spiral.

"Nonsensical. The entire thing, there was no point, no logic, just the setup alone. But there had to be a way, something…" Sherlock trails off again, head bowed to his chest.

"This isn't some… test that you failed, or a game you lost. Moran was never playing by any rules. This is not your fault."

Silence, now.

"Sherlock."

Nothing.

John waits, watching the knots of Sherlock's hunched form tighten and wonders again, more seriously, how long he has been awake for.

On impulse he reaches to touch his head, fingers easing through matted curls and Sherlock makes a low, wailing keen then, hands fisted as he trembles with pent-up feeling.

"I should have known," he gets out, and his voice is high and thick with tears.

"No," John whispers, and puts his arms around him, holding him in. He is stiff still, a twisted-up bundle of raw nerves.

"I'm here," he says again, he can't say it enough now, "I'm here," and with that Sherlock is clutching him, silent and convulsing, tears leaking against his neck.

He has never seen him like this, will never admit how much it frightens him. Sherlock sobs his name just once, John muffled into his skin, and he aches.

"I'm so sorry," he rasps out, and rubs circles against Sherlock's back after that because he doesn't trust himself to speak again.

Sherlock's hands are inside his jacket, braced hard against his shoulder blades, and though his crying abates quickly (too quickly) John can still feel him trembling, hard.

From the corner of his eye, he can just see the blood still on Sherlock's neck, and God they need to get out of here.

"Come on," he says, tugging at his lapels. "Let's go home, okay? Let's go."

Sherlock's unsettlingly pliant now, lets John pull him up and guide him back towards the door with a hand on his lower back.

He pauses abruptly as they near the wall, bloodstains still half-illuminated in moonlight, and John tightens his arm and murmurs "come on," again, just desperate to get him out.

The cab, thank God, is still waiting.

Sherlock gazes fixedly out of the window all the way home, and John watches streetlights pass by over his shoulder and tries hard not to think.