Author's note: This now has a sequel, Crux, and a third part currently being planned.

WARNING: contains triggers for self-harm.

It's mid-winter when Sherlock first notices the line above John's hand, a dark streak on pale skin. It's late evening and they have the heating on full, because John's insisted that's what human beings do when it's cold outside, just like human beings apparently go shopping on Sundays and clean their apartment and make dental appointments and put effort into so many other mundane things. Sherlock doesn't really notice the temperature unless it's relevant, so he more or less gives John total control of the thermostat, and that works for both of them.

He supposes it must have gotten too warm, because John pushes the sleeves of his jumper up slightly. He's sat with his back to Sherlock, who's vaguely watching some awful television program. It serves as a kind of white noise, allowing him to fine-tune his thoughts. Blocking out the humdrum leaves him hyper vigilant to any movement, so he notices John's arms shifting and tilts his head around to look. John doesn't see his face crinkle in confusion.

"What's that?" John jumps at the sudden noise, though he tries to hide it.

"What's what?" John asks, pushing his sleeves back down in an absent-minded manner. It's not an action that Sherlock has ever paid particular attention to. He makes a quick mental note to carry out more research into autonomous behaviours involving clothing.

"Your hand," Sherlock asks, gesturing with the television remote.

"What about it?"

"The scar."

"Oh, that?" John glances down as though he can see through the material. "Bad cut from barbed wire in Afghanistan. It healed up nicely, but it left a mark. Why?"

Sherlock doesn't bother replying, instead turning back around as though he's already bored of the conversation. John resumes typing, and Sherlock is sure that 'human beings' would not have noticed the fragment of panic that his question had induced. He plays the video back within his head, freezing the frame, zooming it in. Yes, a clear split second of something less than fear but more than wariness in John's eyes. "What's what?"

Interesting.

A spot of business with a Japanese man and a blood-soaked hairdryer means that Sherlock can't investigate right away, but he doesn't forget a thing. It's all tucked away in his neat, mental filing system, just waiting for the right moment to emerge. After he solves the case and John goes out to buy milk for celebratory tea (they are British, after all), the enquiry resurfaces. Sherlock takes the opportunity to rifle through John's things.

It's far from the first time he's done it and he has no intention of making it the last- you can discover many things about a person from their drawers. This most recent piece of research confirms his hypothesis: every item of clothing John owns is long-sleeved. It's not like Sherlock's got a huge collection of t-shirts, so there's nothing concrete to indicate that it's suspicious. He makes a note of it anyway and ensures that the clothes are back in order, seemingly untouched, when John returns from the shop.

There are many ways that Sherlock could approach this, and he debates the sneaky and the technological before deciding on the simple. One Sunday, he waits until he hears the shower stop and John get out. Sherlock opens the door with no real warning or ceremony.

(Of course he has a key that fits the bathroom lock. He has a key that fits every lock.)

John shouts, and there's lots of 'Sherlock!' and 'bloody hell' and quite a few words quite a lot stronger than 'bloody' or 'hell', but Sherlock isn't paying attention. It only takes John a second to twist his arm so that it's hidden behind the towel, but one second is too long, and Sherlock sees them: rows upon rows of cuts.

Some are pale and faded, some are dark and healing, some are red and raw looking, and Sherlock finds them a little nauseating to look at for some reason. He can cope with blood and gore- at times he rather enjoys it- but the seemingly endless marks set into John's skin are something altogether different. John doesn't notice Sherlock noticing, so Sherlock grabs something from the shelf, declaring that the object is of 'extreme importance' and sweeping out. It's a small cactus, and he drops it in the dustbin after the bathroom door slams angrily behind him. He stands rooted in place, and thinks. He had not expected this.

It's been over a year since he met John, and Sherlock had thought he knew everything. Everything important, at least. He knows John's shoe size and that his aunt had dementia and that his sister is seeing a new woman, named Liza, who John doesn't really like but pretends to in the hope she might help Harry get better. John hasn't told Sherlock any of these things, but he hasn't needed to. That's how they work- Sherlock deduces things, and John goes along with it.

Sherlock doesn't have a name for what him and John are or what they have. He doesn't have the correct words to describe how it feels to know that John's been hurting himself, or that Sherlock never noticed, or that John never told him. He doesn't know what 'human beings' would do in response to this.

He does the logical thing and does not eat, sleep or speak for seventy-two hours.

He spends most of the time sitting in his room with his feet propped up against the wall, plucking idly at his violin and ignoring anything that John shouts through the door (Sherlock imagines his sudden unwillingness to intrude is a passive-aggressive message about respecting privacy). Sherlock lies back and thinks of lots of clever things that have nothing to do with flatmates or cuts or fear in eyes.

Sherlock doesn't mention anything when he returns to the land of the living, and John doesn't press him. He does smile, though, when Sherlock finally says something ("don't touch the cartilage, it's an experiment").

"You're back, then," John says- and he seems happy, genuinely happy.

"I wasn't aware that I'd left," Sherlock replies, pushing John away from the fridge to peer inside. "Do we have any milk?"

"It's gone off." Sherlock glares. John shrugs. "If you milk, get it yourself. Borrow some from Mrs Hudson."

"She's changed her locks," Sherlock says vaguely, letting the fridge door click shut.

"I might do that myself," John mutters, but he doesn't elaborate. He clears his throat awkwardly. "I'm sorry for… before. For the shouting." There's a short pause. "And for calling you a fucking prat," John adds.

Sherlock's mouth twitches. "So you take it back?"

"No, you're still a fucking prat, but it rude to let you know." And just like that, the world seems right again. John spends the evening replying to comments on his blog and Sherlock makes more notes on tobacco ash (even if he doesn't upload them anymore).

As he sketches, his mind drifts back to its current favourite topic. Sherlock wonders why any of it matters anyway. John self-harms. It's hardly an uncommon problem, even if it isn't one Sherlock had been prepared for. Sherlock curses himself for that. He hadn't expected this because he had assumed John wouldn't do that kind of thing, and detectives can't afford to make assumptions. People often think that's that all he does, but that's like saying mathematicians assume that ten plus ten equals twenty. Presumption is dangerous and it leads to overlooking things like a flatmate's potential psychological issues.

John does have post-traumatic stress disorder- albeit an unusual kind, but there's no doubt it's there with that level of flashbacks. His sister is an alcoholic and he has psychosomatic disabilities. There are good reasons that John was seeing a psychiatrist when they first met. Sherlock had always thought it was for PTSD, what if it was also more to do with the fact that John copes with issues by bleeding himself? There is absolutely no reason to imagine John slowly pressing a blade into his skin, but the image won't leave Sherlock's mind. He plays the violin at full volume until four A.M, until the sound starts to cloud the picture.

Why should it matter? Everybody has their own coping method. Harry drinks, Mycroft eats, and Sherlock injects or inhales substances of varying legality. From what he's seen, John's wounds don't appear too deep or too numerous- certainly not life threatening. Logically, it shouldn't matter in the slightest.

But it does. It does, so much that it starts to wake Sherlock up at night. He realises one night that the reason it's bothering him so much is that he didn't notice. He didn't pick it up. He was wrong. He realises the next night that it's because John didn't trust him enough to tell him, that John doesn't value him enough to confide in him. He realises a few nights later that it's the primitive annoyance of empathy- he does not like to imagine John feeling that level of emotional pain. It doesn't take long for Sherlock to realise that it bothers him for many, many reasons, and that each reason is the most important one.

Watching John buy a new razor in the shop one day and being hit with the realisation that he'll probably use it to open up his skin hits Sherlock like a wrecking ball. He has never felt more pathetically human in his life. He has never felt more in his life.

It's not all doom and gloom; they're still as they always were. Sherlock makes jokes about Mycroft's sexuality and his weight, and John laughs and encourages him to steal ashtrays. There are many, many days filled with action and laughter and enjoyment. Sherlock certainly doesn't forget about it (he doesn't do forgetting), but the obsessive uneasiness occupies less of his time. It's only after the business with Irene Adler that the issue rears its ugly head again.

He doesn't intend to walk in on John, not really. Maybe on some sub-conscious level, he'd remembered they'd had a hard day with their newest case, and that Harry had fallen off the wagon again, and that Anderson had implied several unkind things that hit John harder than he cared to admit. Maybe he had realised that two plus two makes four and that John had retreated to be alone earlier than usual. Either way, all Sherlock is thinking when he barges into the other man's room is that the length of the grass in the park could prove their client is innocent, and that John will want to help him find out.

Sherlock's halfway through a sentence when he sees John, and the offer dies on his lips. John's sleeve is pushed up and, on his arm, a new cut has blossomed. A razor- the same one Sherlock watched him buy a few days ago- sits nearby, glinting mockingly at Sherlock. John's lying on his bed, staring up at the ceiling, a thin line of blood trickling down his arm. He bolts upright when he hears Sherlock, his panic evident, but the fight seems to leave him as soon as it came. Only a moron would try and lie to Sherlock. John twists and props himself against the wall instead, like his body is too heavy to work properly, like it's too much for him to command.

There are a few seconds of awful, endless silence.

"I'd guessed, you know," Sherlock says.

"I know," John replies.

Sherlock had been ready to leave, but for reasons he can't name, the response stops him. It helps that John doesn't cry, or shout, or throw anything. He just sits there, not looking at Sherlock, and Sherlock leans against the doorframe and does not look at John.

"Not very long ago, though."

"I know."

"Is this just one of those things human beings do?" Sherlock asks, and he supposes it's meant be funny. He sees John smile out of the corner of his eye, but he doesn't look happy.

"No. Not most, at least." Sherlock nods. "You can sit down if you want." Sherlock does, sitting cross-legged on the foot of John's bed. He leans back against the same wall.

"Do you want a bandage?" Sherlock asks, still not looking over at John. He's well-acquainted with their first aid supplies- last weekend alone produced four separate third-degree burns, as his work with lithium tends to.

"No, it'll be fine. It's not deep." That's true. Some of the others look like they have been, but today's cut is shallow.

"So how often does this happen?" Sherlock doesn't specify what 'this' is, and John doesn't make him. Sherlock's never seen John like this before, so flat and apathetic. He decides that he doesn't like it.

"It's not exactly a routine thing," John says. "When I need to, I guess."

"Interesting vocabulary choice," he murmurs.

"It fits." Sherlock doesn't say anything, because he learned a long time ago that sometimes the best prompt is silence. John takes the bait. "It started when I was thirteen, I think. It's been on and off since then. It depends on how things are."

"Alright." And then, a few minutes later: "Then how often is it at the moment?"

"Once a month, maybe. It depends." Sherlock would ask what it depends on, but he's not sure that he'd understand or that he wants to try. He doesn't even mean to ask the next question.

"Is there anything I can do?" Sherlock feels John look at him then, and he slowly turns to do the same. Their eyes meet, and Sherlock holds the gaze without blinking.

"I don't think so," John says eventually, softly. Sherlock nods and looks away again. They sit together wordlessly, and a little while later Sherlock leaves. The blood has dried on John's arm by the time he finally cleans the wound.

Sherlock learns to read the signals after that, slowly but surely. He learns that, the morning after a cut (because, for some reason, it always seems to happen at night), John is dulled. He is slower to react, like he's caught up in his own world. He tries to avoid looking Sherlock in the eye, and when he does, there's an aura of guilt and shame.

Sherlock may be able to detect it, but he's not sure what to do with the information. Announcing it as he would most deductions feels somehow wrong, so he tends to pretend everything is fine and continue with life as usual.

One morning, he's working when John walks in with that look on his face, and Sherlock decides to go ahead and mention it.

"That's the third time this week. Everything okay?"

John starts to deny things, to ask Sherlock what he's talking about, but Sherlock looks up at him and John stops. He knows there's no point.

"Family problems," he says briskly. "Nothing significant."

If Sherlock was somebody else, he'd ask questions or get upset or tell John that he was there for him. Instead he nods, picks up a newspaper, and says "Remember to sanitise the area afterwards if needed. You don't want to develop blood poisoning." And if John was somebody else, then he'd ask questions or get upset, but instead he nods and makes another mug of tea. Sherlock's not sure quite how he feels when their stock of disinfectant wipes begins to deplete.

The frequency isn't consistent. Some months, every week has two or three of what Sherlock has dubbed 'aftermath days'. Some months, there are none at all. Those months are better- John is lighter, happier, somehow more present. It's only when the fog that clouds John's eyes is absent that he really seems to exist, and to seem happy that he does so.

"I'm not suicidal, you know," John says one day. One look at John had told Sherlock it was an aftermath day, and one look at Sherlock had told John that he knew. Sherlock digests this piece of information with no apparent distress or concern.

"I know," he replies. "I'm relatively confident I would have picked up on that. If you were."

"Fifty per cent certain?"

"Ninety-five."

"That's 'relatively confident'?" John laughs.

"It's about as unsure as I get." There's a silent sorrow underpinning Sherlock's comment. They both pretend not to notice.

"It's just… what I do," John waves his hands, trying to indicate what he doesn't know how to say. "It's what I've always done. I don't even know how I'd go about stopping now- it's automatic." Sherlock's brain helpfully offers up lectures on neuroplasticity or the cognitive approach to psychopathology, but he shoos them away.

"Would you want to stop?" he asks curiously. John thinks about it.

"Did you want to stop taking drugs?" Sherlock thinks about it. Neither of them manages to produce an answer, so they move onto their most recent case instead.

There's an aftermath day when they get back from Baskerville, and it's a bad one. Sherlock isn't shocked when he realises, but it hurts all the same. John won't say and Sherlock won't ask, but they both know it's to do with what happened in that laboratory. John had been in danger, afraid, helpless- out of control. It all seems so obvious in retrospect. Sherlock wonders why he still hasn't developed a technique to recognise when John is feeling vulnerable; perhaps he hasn't wanted to try. Perhaps he's been too afraid of finding there is no way.

A few days after they return home, John seems tense, on edge. Harry's broken up with her most recent girlfriend, and John has been systematically ignoring her texts for four hours. The nineteenth time John's phone buzzes, something seems to topple inside of him. Usually, Sherlock probably wouldn't even notice, and it's only when he remembers that it was John's turn to buy milk and thinks of what else he could have bought that things click neatly into place. John's about to leave, but before he can Sherlock is upright too.

"Come with me." Sherlock grabs his coat from where he's thrown it over a chair and pulls it on.

"Sherlock, I'm busy."

"We're going out," Sherlock says, ignoring him.

"Where?"

"It doesn't matter." John is so confused that he lets himself be pulled out the door. They end up in some awful twenty-four hour café, and although Sherlock says it's because he had an important theory about salt cellars, it isn't. It's nearly one A.M before Sherlock makes the careful judgement that it's okay for them to return home. They get a cab back and John's hand slips silently into Sherlock's as they drive. Neither of them says anything to acknowledge it, and they release each other without a word when they pull into Baker Street.

Sherlock knows that you can't save people. He meant what he said- he isn't a hero. They don't exist. He's not a naïve little girl, telling people that they're beautiful and loved and expecting their psychological problems to instantly shimmer out of existence. But the morning after they return home from the café, when John wanders out of his bedroom yawning, Sherlock can't help the slow smile that spreads over his face. He doesn't expect to break a habit that's over twenty years in the making. He doesn't expect great things. But today is not an aftermath day, and that's good enough.

Moriarty is arrested, and Sherlock is called to testify. He and John stay awake for almost the whole night before the trial, letting the television blare and talking about nothing in particular. Sherlock has never met anybody like Jim Moriarty, and there are so many unknown variables that he can't predict what's going to happen. The anxiety in his stomach is unfamiliar and uncontrollable, and he can see that John is feeling the same way. When 'that' comes up, he's almost grateful for the distraction.

"I still don't understand why you didn't tell me," Sherlock says irritably. "It would have saved me a lot of investigation."

"You're a detective, Sherlock. If people just told you things all the time, you'd die from the frustration." This is true.

"Yes, but this isn't a case."

"All the same-"

"And potential flatmates should know the worst about each other," Sherlock adds. He hadn't meant for it to hurt John, but he can see quite clearly that it has.

"I did say that nobody would want me for a flatmate," John says, in a joking tone that doesn't quite hide the bitterness, and sudden realisation dawns.

"Oh," Sherlock breathes. "You think this is embarrassing."

"Of course it's embarrassing," John says incredulously, like he can't believe Sherlock could think differently. "I'm a fully grown man with a fifteen year old girl's problem."

"You know fully well that that isn't true."

"Come on, Sherlock. Who the hell would want to know somebody like me?"

John has nearly forgotten his question when Sherlock replies, almost an hour later. Sherlock has tried nearly every combination of words in the dictionary to sum it all up, and he's still not sure that he's managed it.

"I would," he says.

John doesn't reply, but he seems to relax, and when he speaks again he sounds somehow lighter.

"I didn't tell you," John says, "because I've never told anybody." Sherlock knew this already- had picked it up from a million different subtle clues- but he plays along.

"What about Ella?"

"God, no. She's an awful therapist. Even I know that."

"But why not tell anybody? Ever?" Sherlock isn't used to asking things like this. He can interrogate, yes, but a conversation where he asks questions for no personal or professional gain is unfamiliar territory. He is playing at being human and finding that it feels right, somehow.

"There's never been a reason to. Nobody's ever guessed, nobody's ever asked. Nobody's ever really wanted to know." The 'until now' remains unspoken but hangs in the air, more obvious than anything said aloud. John looks at Sherlock, and smiles. It's a smile of gratitude, and kindness, and something else that Sherlock can't name. A strange kind of warmth lights Sherlock up from the inside (and although he'll never know, the exact same thing happens within John).

The Moriarty case builds and people begin to whisper things about Sherlock. Fake, liar, fraud. He wonders if John doubts him, is convinced of it, but John insists otherwise and Sherlock believes him. One evening, halfway through a completely unrelated discussion, John looks Sherlock in the eye and tells him "I trust you, you know." The comment knocks the words out of Sherlock's mouth, and he has no choice but to dumbly wait for John to continue.

"You know I do," John says. "I wouldn't have told you all I have otherwise."

"You didn't tell me anything. I caught you." Sherlock is being unfair and ruining what is supposed to be a good, reassuring moment, but unfairness and ruin is all that surrounds him right now.

"Sherlock, if I hadn't trusted you, I wouldn't have stuck around when I realised you could open all the doors. Which, for the record, took two days."

Sherlock considers this.

"Can I see?" he asks, uncharacteristically softly. John hesitates, then slowly unzips the jacket he's wearing. He lets it fall to the side, and Sherlock sees that he's wearing a plain grey t-shirt. It must be new- judging by the colour intensity and the state of the fabric, it's three weeks old at most. It is John's first short-sleeved shirt in ten years. That means something important to both of them.

There are more cuts than Sherlock can really take in on John's left arm. They're mostly over his bicep, but when John pushes back the material they cover his shoulder and trail all the way down to his wrists. Most are old and fading, but some are brighter and reluctant to leave.

"None of these look new," Sherlock comments.

"No. Not since Baskerville." That's over three months, and Sherlock can't help but feel a small tug of pride for John in some deep, forgotten centre of his soul. Sherlock reaches out slowly, and John lets him. He stays very still as Sherlock brushes his fingertips gently over the scars.

"I hate them," John says bitterly. "They're disgusting."

"Don't be an idiot, John."

"They're embarrassing."

"No, they aren't."

"They are, Sherlock. They're like reminders of every single time I screwed something up."

"You have a scar from Afghanistan on your shoulder, but you aren't humiliated by that. I don't see why these should be any different."

"Because they aren't the same." John sounds exasperated.

"Why not? They're all battle scars."

"Oh, come on. I did these to myself. Nobody fought me except me."

"And isn't that the worst battle of all?" Sherlock murmurs.

They don't talk anymore about it, not that night. And if their hands knit together or they curl into each other (in that way that lets comfort and trust and that emotion Sherlock still can't name wrap around them), then that's their business and nobody else's. It's certainly something that neither of them is prepared to be ashamed of.

The conversation on the rooftop nearly kills Sherlock. When he says goodbye, he knows that it's probably not for the final time. He knows that he will come back if it all works out, and he's ninety-eight per cent confident that it will. All the same, it's still a goodbye. He still steps off the building, and John breaks at the same time that he does.

John's fingers grasp clumsily for his wrist where, if Sherlock hadn't been ever so clever about all of this, he would find a pulse rate elevated from something that isn't adventure or adrenaline. Instead, he finds nothing.

After Sherlock has stolen away from the hateful scene, giving his name to an alien's corpse and leaving John wrapped in the arms of strangers, he gives himself time to do what he has to. In a dumpster four streets away, Sherlock leaves a plastic bag with the handles tied tightly together. In it is every razor blade, every pair of scissors and every knife from 221B Baker Street. It won't be enough, and Sherlock knows that. His logical side tells him that it probably won't make any difference at all but, for once, Sherlock doesn't give a damn about logic.

If the conversation killed Sherlock, then John's speech at his grave either brings him back to life or destroys him completely; Sherlock can't tell which one. He watches John from his hiding place, and he hears every single word though he can't see very well. He only gets a good look when John turns to leave and pain stabs Sherlock through the heart, because there is no doubt that today is an aftermath day. Sherlock can't stop the tear that rolls down his cheek. When he looks at John, his own shame and guilt and that feeling that he has finally, finally found the word for are so strong that it hurts to breathe.

Heroes don't exist and if they did, they wouldn't fly off buildings and tell the world they fell.

One day, he convinces himself, they'll meet again. One day, when the world is brighter, Sherlock will walk into 221B Baker Street or wherever John has moved to, because anywhere he is will be Sherlock's home. And maybe they'll pick up where they left off, or maybe they'll move it further, or maybe they'll push it away and never ever speak of it again. They'll solve crimes and John will blog about it, and whether or not Sherlock's trousers are involved won't really matter.

Sherlock still isn't naïve enough to think that he can fix all of John's problems. He's just a human being, after all, and it's neither his responsibility nor his duty to try and stop John from hurting himself. But he still wants to be there, because it turns out that what he offers and what John needs fit together like their fingers in the dark. Cutting and jumping and falling and hurting may only take seconds, but the effects last so much longer. Sooner or later, Sherlock will find his way back to John- because whilst he can't reverse the pain, he can try and help with the aftermath.