Sherlock hides it for as long as he can.
But John is a doctor. He's trained to notice these things, and notice he does. It starts off with small things, things John puts down to stress (as Sherlock hopes he will). Forgetting crucial pieces of evidence, getting distracted in the middle of conversations, struggling to remember a suspect's name. John finds it unusual, but puts it to the back of his mind. They've been working hard lately, taking on more cases than usual.
Sherlock is desperately trying to cram more information into his brain, trying to replace the things he is losing.
They are in the middle of breakfast when John gets a text from Harry. He reads it and comments to Sherlock, "Harry's got a new girlfriend."
Sherlock glances up from his newspaper. "Who, sorry?"
John frowns. "Harry."
Sherlock looks at him blankly, lips moving, mind racing, trying to place the name before John realises anything is wrong. He runs through their acquaintances at the Yard, through friends John may have introduced him to - /LestradeDonovanAndersonSarah MikeMollyNO/ - and comes up blank.
"My sister, Sherlock," John says gently. "My sister Harry."
"Of course. Your sister Harry." A ball of ice is forming in Sherlock's stomach. John has told him about his sister, many times. Sherlock has met her. She teased the two of them about having sex. He remembers it very clearly - specifically the tortured look on John's face. It was quite delightful.
"Harry," he says to himself again, and looks up when John's fingers brush his.
"Sherlock. Is everything alright?"
John's face shows nothing but concern. His eyes take hold of Sherlock's and fix them in place. As much as Sherlock would like to look away and avoid this conversation, John's eyes transfix him and he finds himself saying, in a voice smaller than he thought possible, "I forget things."
"Everyone forgets things," John says. John trusts in the normalcy of things, in averageness, in regularity. He believes firmly that nothing is wrong with Sherlock, just because that is the way things have always been.
"I don't." Sherlock is adamant. He snatches his hand away and occupies it with a piece of toast he does not want and will not eat.
"Sherlock - "
Sherlock flips his newspaper up in front of his face. He doesn't read it.
"Sherlock. Don't do this." John is pleading. Sherlock is incapable of having this conversation. He gets up and walks into the bedroom without another word.
That afternoon, the pair are summoned to Lestrade's office. As they head out the door, Sherlock picks up a notebook and wordlessly hands it to John.
As Lestrade runs them through the details of the case - missing children, suspicious circumstances, /locationsdatesnamesdata/ - Sherlock touches John's arm and murmurs, "I need you to write this down."
Lestrade pauses mid-sentence. "Do you need me to repeat something? I can start over - "
"It's fine," Sherlock assures him as John scribbles in the notebook. "John is more than able to cope."
When Lestrade's briefing is over, Sherlock takes the book from John and reviews his notes. John's handwriting is a typical doctor's, barely legible, but Sherlock has grown accustomed to his scrawl. As he reads over the facts of the case, committing each word to memory as if recording a sacred text, John leans on Lestrade's desk and watches, frowning again.
/Maisie Jones 8 years old last seen 23 March Rosa Howard 6 years old last seen 6 April Anthony Johnston 7 years old last seen 15 April./
Sherlock looks up from the notebook, reciting the list to himself. Three names, three ages, three dates. This is child's play, for Christ's sake, it can't be that hard to remember. The first - /M something/ - the second began with S, he is sure of it - and the third he can't recall but two is enough to be going on with, surely.
"Sherlock."
John's voice cuts through Sherlock's thoughts. The doctor is standing in front of him, now, gently taking the notebook from his hands, squeezing his fingers. "I'll remember for you."
Sherlock nods curtly. It's all he can do.
Sherlock solves the case - or, more accurately, John solves the case with excessive prompting from Sherlock. It takes two days. In that time Sherlock doesn't eat or sleep. By the time the abductor is delivered to the police, by the time the Maisie and Rosa and Anthony are returned to their parents, Sherlock can barely stand. His brain is a mess of long-forgotten names and places and, try as he might, he still cannot recall the name of John's sister. This one fact stands out as singularly important to him, and he mulls it over as John drags him back to the flat, as they eat their Chinese takeaway in silence, as he follows John to bed and collapses without taking off his shoes.
John sighs and tuts fondly. "You can't sleep in your shoes, Sherlock."
"Can," the greatest mind in England mumbles sulkily into the pillow.
"I'm not taking them off for you," John says. Sherlock can hear the smile in his voice and cannot remember his sister's name. He makes no move to remove the offending shoes.
When he hears John sigh and kneel down, and feels his shoes being gently slipped off his feet, his mind is engulfed in a cold wave of panic. He can do that himself. He has always done it. If John does it now, Sherlock will forget how to.
But then John peels off his socks and kisses his Achilles tendon and Sherlock thinks that, maybe, having John take off his shoes for him would be alright.
Months go by. John accompanies Sherlock on cases and writes down facts. Sherlock relies on John, needs him, more than ever. John understands this without being told. He slips the correct names and dates into Sherlock's sentences so seamlessly that they almost become one person, Sherlock's mind being propped up by John's. It takes Lestrade a little while to notice, but when he does, he takes John aside and the two of them speak in hushed, worried tones that Sherlock strains to hear.
Later, after the case is successfully dealt with, when they are sitting on the sofa together, an empty pizza box in front of them, Sherlock slouches and curls his knees up in front of him and asks what John and Lestrade talked about.
"He's worried about you. I'm worried about you as well."
"I'm fine." Sherlock has never told so obvious a lie. He stares at the wall in front of him and ignores the pain he knows is sketched across John's face.
"You're not fine," John says. "You're obviously, completely not fine, Sherlock. We're concerned about your health - "
"/Sod/ my health," Sherlock bursts out, making John flinch. "My health is fickle and demanding and constantly deteriorating, and I can't /think/ properly anymore! My mind is a fucking seive, I can't remember what we did yesterday, or what was on that pizza, or what your sister's name is."
John is silent. Sherlock glares at him defiantly, wishing perversely for him to be upset, as if it will somehow lessen Sherlock's own agony. John drops his head into his hands, and rubs his eyes. He looks old, Sherlock realises suddenly. He has been aged by the demands of living with Sherlock. Perhaps it would be better if Sherlock fell to pieces alone.
John looks up at him with damp eyes. "I want you to see a doctor."
"I have a doctor," Sherlock mumbles. He tries to take John's hand, knowing that if he can just do that everything will be alright again. John pulls away.
"A proper doctor," he says. "I'll go with you. But I need you to see someone. Will you do that? For me?"
Sherlock stares at the wall for a long time. The yellow face he drew there (that was such a long time ago, wasn't it, that he drew that face and shot at it and shot at John) leers at him, mocking him with time and with memory he does not possess. Finally Sherlock nods.
John reaches out and holds his hand, but nothing gets better.
They visit an old medical school colleague of John's, a psychiatrist, who tentatively diagnoses Alzheimer's disease. Sherlock stares at the floor, stony-faced, and John waits until Sherlock has left the room before he cries.
They return home in silence. John tips the cab driver extra. Sherlock knows that John has been crying, but doesn't say anything. He knows that if he does, John will cry again, and Sherlock would rather forget a thousand names than see John cry.
They walk inside. John stands in the doorway, unsure. Sherlock drops his coat on the arm of the chair and opens John's laptop. He reads two paragraphs of Alzheimer's symptoms before John puts a hand on his shoulder and tells him to stop.
"I want to know what's coming," Sherlock says, and John blinks forcefully at the tears that keep threatening to invade his vision.
"Stop crying," Sherlock says, but John can't.
/Don't cry,/ Sherlock tells himself, but his discipline is apparently fading as fast as his memory, and by the time John pulls him into an embrace his face is already damp.
The years pass. Sherlock's thoughts become more disordered. John takes care of him, as John always has. Sherlock is grateful and sullen.
Sometimes, Sherlock forgets. Always, John helps him to remember.
The terrible tragedy of the situation lessens as the minute everyday frustrations become more apparent. Sherlock gets angry at himself when he can't tie his shoes, when he forgets experiments, when he introduces himself to Anderson, the man on Lestrade's team who he can't for the life of him remember having met before.
John is there to pick up the pieces, to tie Sherlock's shoes and button his shirt, to clean up the sticky mess of chemicals from the kitchen table, to insult Anderson in Sherlock's place. He's not as good at it as Sherlock, but he tries just the same.
One morning, after a better week than they've had for a long time, Sherlock gets up and makes toast. John is proud of him, but keeps the thought to himself. The last thing Sherlock needs is to be treated like a child.
The two of them sit at the table in comfortable silence, a silence which is broken by the buzzing of Sherlock's phone. He reads the message.
He reads it for a long time.
Finally, he says, "John."
John looks up to see Sherlock frowning at his phone, his hand just barely shaking.
"John, I need you to send a text for me."
Sherlock's head drops as John pries the phone from his white-knuckled fingers. John doesn't say that everything will be alright. Everything hasn't been alright for years. Sometimes, John can be as forgetful as Sherlock. The only difference is that John is wilfully so.
John waits for instruction, fingers poised over the phone's keypad. When Sherlock finally speaks, his voice is cold and thin.
"Tell Lestrade I won't be accepting any more cases."
They stay in Baker St. Neither of them can bear to leave.
John reduces the number of hours he works at the surgery. He knows, eventually, he'll have to leave entirely.
Mrs. Hudson agrees to keep an eye on Sherlock whil John works. She sits in the lounge, knitting, chatting happily away to him while he reads, plays the violin, lies on the sofa staring at the ceiling and trying to remember the name of John's sister.
Sherlock asks John several times who the old lady is that keeps talking to him. John doesn't have the heart to repeat this to Mrs. Hudson, but he suspects that she already knows.
John works in the mornings, when Sherlock is at his best, and comes home in the afternoon, ready for the evenings when Sherlock's mind bends out of his control and John is the only one who can calm him.
It is on such an afternoon that Sherlock frightens John for the first time.
John reaches the top of the stairs, announcing that he's home, just as he's done for the past year and a half. He takes off his jacket and leaves his keys on the table in the lounge.
"Sherlock?"
John checks the kitchen. Lately Sherlock has been conducting experiments again, just simple ones, things that don't require much effort or waiting or forgetting. John has often come home to find Sherlock sitting at the kitchen table with a delighted expression on his face, as he watches chemicals and formulae dance a convulted jig across a petri dish.
He isn't there today.
John walks towards the bedroom with a heavy heart, calling Sherlock's name, hoping that Mrs. Hudson isn't naive enough to let him go out on his own.
Sherlock steps out of the bedroom. He holds John's gun in his hand, loosely, as though it is a hairbrush or a newspaper.
"Sherlock," John says, quietly. "Can I have that?"
Sherlock stares at him coldly. "You want me to give this to you? And leave myself completely defenseless? I am not an idiot."
There is no warmth in Sherlock's eyes, no recognition. John's heart appears to stop beating.
"I - Sherlock, it's me. It's John. You know who I am," John says. He is not pleading, not yet.
"I can confidently say that I have never seen you before in my life, and that you seem to think so is an insult to both our intellects," Sherlock responds, raising the gun. "I suggest you leave now, or I shall be forced to use this, a situation I am sure neither of us would be entirely happy with. The cost of cleaning blood from carpet is positively horrendous."
John raises his hands. "Alright. Alright. I'm going. I'm going to walk back down the hallway now, alright? If you put that gun down, I'll go. That's a promise."
Sherlock slowly lowers the weapon, maintaining eye contact with John. His eyes are brilliant, focussed, shining. Everything that Sherlock no longer is. John can feel tears pricking at his eyes as he slowly walks backwards, away from Sherlock. His Sherlock.
Sherlock glares at him. John turns and walks into the kitchen, sitting down just out of Sherlock's line of sight, and prays to a God he does not believe in that Sherlock will recognise him next time.
"I forgot you," Sherlock says to John, later, clinging to him. "I won't do it again. I promise."
John nods and holds Sherlock and forgives him, knowing that this isn't enough, that Sherlock cannot forgive himself.
The next day Sherlock makes John stay home from work so that they can sit opposite each other in silence while Sherlock tries to memorise every inch of John's face, every line, every pore. He tries to engrave the sight on his memory so that next time, he will know.
John feels a tear slip down his cheek after an hour. After two hours, Sherlock reaches out and takes his hand and holds it until both men's fingers are numb.
After the fourth hour, Sherlock slips forward off his chair and rests his head in John's lap and goes to sleep.
John's tears adorn Sherlock's greying hair like crystals.
Another year goes by.
Sherlock forgets.
John remembers.
Sherlock opens the drawer of their bedside table, one day, while John is out getting milk. There is a gun there.
Sherlock has been planning this for months.
He knows there is something he should do with the gun. Something important.
Somehow, the gun is intrumental in making sure Sherlock cannot see John's tears.
Sherlock must do it before John gets back, he knows. The plan doesn't work if John is there.
He stares at the gun and his reflection stares back from the dark metal.
After a few minutes, Sherlock gives up. He will remember tomorrow, he is sure of it.
After a while, a man in a cable-knit jumper brings him a cup of tea and kisses his forehead.
Sherlock smiles at the man and thanks him. He assumes the man will introduce himself, but instead he simply stares at Sherlock for a moment, and then says he will be in the lounge.
Sherlock nods.
He hopes he'll see the man in the jumper again.