It starts slowly.

John begins misplacing things. Little things, innocuous things really, like his keys or his phone or his wallet. He'd set them down somewhere and lose track of them completely. It was nothing a million other people in London didn't do a thousand times a day.

Nothing to be overly concerned about.

Nothing at all.

Then, one day, he forgets he turned the kettle on to make tea and nearly burns the flat to the ground.

No matter how he tries he cannot extinguish the flames which rampage through the kitchen, feeding on the chemicals Sherlock has stored there, and he's forced to drag Mrs. Hudson out onto the street to escape the danger. The fire trucks arrive, sirens wailing, followed by Lestrade and some of The Yard.

"Heard there was a fire," the Detective Inspector says. "One of Sherlock's experiments?"

"No, it was me. Left the kettle on."

"Bloody hell…"

"I know," he says. "I know."

"No, I mean, look!"

He follows the line Lestrade's finger makes until he sees his flatmate's lanky form striding across the street, coat flying behind him dramatically, his face wide open with worry. "John!" he calls. "John! Are you hurt? Are you alright?"

He find himself being manhandled infront of God and everybody as Sherlock turns him this way and that, examining him for any injuries. "I'm – Christ – I'm fine, Sherlock! I'm alright!"

"What happened? Was it Moriarty? I knew he couldn't be dead. It was too easy."

"What? No. Sherlock, it was a kitchen fire. That's all. A simple kitchen fire."

Sherlock steps back. "But you're not hurt? Mrs. Hudson's not hurt?"

"No. We're fine. Got out in time."

"I see," he says. "Well then. John?"

"Yes?"

"How could you be so stupid?"

"John, what's the maximum dose of morphine an adult male my size can have within six hours?"

"I…I don't remember."

"Fucking hell. It's Wednesday. I was supposed to be at Surgery two hours ago! Sherlock, where are my keys?"

"John, the power is off."

"And?"

"Didn't you pay the bill this month?"

"What is the matter with you?" Sherlock snaps.

"Sorry?"
"You've been bumbling around for months, John, and it's only getting worse. I thought maybe you were infatuated with someone but you clearly haven't been shagged so –

"Jesus, Sherlock, I slipped up a bit. That's all. Living with you is enough to send anyone a bit round the bend, so get off my back!"

It all comes to a horrifying head that Christmas.

Mrs. Hudson has decorated the flat to celebrate the coming New Year and they're throwing a party later that night. Life is quiet, if you don't count the triple homicide and missing Viscount they're investigating.

John is tugging at the Santa hat he's put on the skull when he turns to Sherlock and says, "So has she texted you anymore?"

"Has who texted me?" he asks, eyes still glued to his microscope.

"Irene Adler."

Slowly raising his head, Sherlock turns to stare at John.

"Alzheimers?" he says, staring at the CT Scan hanging on the wall behind him.

"I'm afraid so, Doctor Watson."

"But…that's not possible. I'm…I'm not yet forty!" This is not happening. This is not happening. This is not happening.

"I know. If you'd been older we might have caught it sooner. Someone would have noticed, because we look for memory loss in the elderly, but we tend not to notice in the young. I'm very sorry."

Sherlock is sitting in his chair when John arrives home from his appointment.

"How did it go?" he asks.

John buries his head in his flat mate's lap and cries.

Sherlock buys them a camera and sets it up in various places around the flat. He captures moments, happy/silly/angry/beautiful/wonderful moments. He captures a terrible row between himself and Harry, with John looking on dolefully from the background. He captures John trying to remember the password to his blog, becoming frustrated, and chucking the laptop across the room. He captures breakfasts and lunches and visits from Mycroft. He captures his own confession of love. He captures their first kiss.

He captures all of it. Because all of it is precious.

"No. No way. I am not having sex on camera, Sherlock. End of story."

"John, please."

"This is mad."

"Just once. You won't even know it's there. I'll make sure of it."

He caves, because even when his sanity is hanging by a thread, he'd do anything for Sherlock Holmes.

Perhaps especially when his sanity is hanging by a thread.

"Christ, I can so see this winding up on the internet."

Sherlock hold him close, trembling as their limbs intertwine and their mouths meet frantically. "Don't forget me," he whispers. "Don't forget me."

Sherlock is a passionate lover.

John doesn't think he'll ever forget that, Alzheimer's be damned.

Sherlock rests his hands lightly on John's shoulders and leans down to kiss the side of his neck the way he knows John likes. "Come to bed," he says.

John jerks away. "Jesus, what is wrong with you?" he snaps. "I'm not gay!"

The first time John forgets Sherlock's name is followed closely by the first time John forgets that Baker Street is his home.

In both cases, Sherlock empties his revolver into the wall.

John stops going to crime scenes with him.

He can barely remember where he lives, much less the names of all the Yarders. He's started thinking of them in terms of That-Grey-Haired-Fella, Miss I-Need-A-Good-Shag, and Mr. Someone-Sew-My-Mouth-Shut-So-I-Don't-Drown-In-My-Own-Stupidity.

He thinks their new names are hysterical.

Einstein-With-Cheekbones-And-A-Bad-Attitude doesn't.

John forgets how to tie his shoes.

Einstein-With-Cheekbones-And-A-Bad-Attitude brings home a pair of nice Oxford Loafers the next day.

He's grateful to him, but it seems like a bit much for someone you barely know.

"John, you have to eat."

"I ate already."

"You haven't eaten in days."

"Yes, I did. I ate with Sarah."

"John, Sarah's in Tibet. Has been for two years."

"That's…that's not right…"

"It's fine, John. Just eat."

"Sherlock?" John's voice is soft, barely above a whisper.

A hand touches his in the darkness.

"What month is it?"

"July."

"So much time…"

"Yes."

"Sherlock?"

"Yes?"

"I don't want to live like this."

"You're not thinking clearly."

"But I am. I'm completely lucid. I promise."

Silence reigns for a moment, then, "John?"

"Yes?"

"I don't want to live without you."

It's amazingly easy to find drugs in London if you know where to look.

He stashes his supplies behind a loose tile in the bathroom.

He would wait, until the time was right, and then…

Well. He'd cross that bridge when he came to it.

John is fighting and scratching and biting and screaming in Farsi and Sherlock does his best to hold him and not hurt him, but John is not in London right now.

John is fighting for his life.

Before the Alzheimer's, he could have thrown Sherlock off him as though he weighted nothing more than a flighty leaf. Now, however, his muscle mass has gone and he is at the mercy of his flat mate turned enemy.

He collapses after a while, his energy spent, and then (to Sherlock's horror) he begins to cry.

"I wanna go home," he says.

Sherlock smooths his hair. "You are home, John. You are home."

"I want Sherlock," he says.

"It's alright. I'm here." He presses a kiss to John's temple. "When memory fails, I'll always be here. Don't worry."

He rocks John to sleep to the sound of falling rain.

He slips from the bed and into the bathroom. Pulling his supplies from their hiding place, he mixes a cocktail in a syringe, and returns to the bedroom.

"I love you, John."

"I said it, didn't I?" Sally Donovan says, standing in the middle of 221B looking entirely too smug. "'One day he'll cross the line. One day we'll be standing around a body, and Sherlock Holmes will be the one that put it there.' But he's not one to take things the normal rout, is he? No. Had to go the extra mile, he did. Murder/Suicide. Can you belie–

"Sally, shut up," Lestrade says. "Just…just shut up."

He watches from the window as Sherlock Holmes and John Watson are loaded into the ambulances below, encased in heavy black bags, taking on death the way they'd done everything else.

The Detective and The Blogger.

Together.