The case is solved by nine p.m., but it is two in the morning before Joan and Sherlock have the kidnapped sisters, the man who took them, and the police, all in one place and in what Sherlock sarcastically refers to as an optimum configuration. The NYPD took an age to respond to his messages, and he and Joan found themselves repeatedly at the wrong end of a Glock before Joan created a distraction and Sherlock was able to overpower the kidnapper.

Bell and Gregson, badges and guns out, are morose and snippy as the paramedics tend to the victims and investigators set up painful floodlights in the chilly warehouse where the teenage girls were discovered. The police have good reason for ill humour: the presidential candidates are due for a New York visit at the end of the month, and crimes like these detract from the hoopla. The NYPD needed a fast resolution, and it is too soon yet for simple relief that Sherlock has once more provided one.

Sherlock is wired, pacing around and finding more and more evidence the police ought not to have missed the first time they searched this wharf.

Joan sits on an upturned crate and wipes her hands with the towel the medics gave warehouse is dark and damp with October cold. Bales of rag are stacked everywhere for shipping to China. The cold concrete floor is stained and filthy, and the night air leaks under the corrugated walls, carrying moisture and desolation straight from the East River and onto the back of Joan's neck.

She scrubs her hands and applies anti-bacterial gel. Those kids will be all right. They are seventeen and nineteen and they have a loving family who will protect and support them. They have been lucky not to join the earlier victims, whose bodies are even now in the police morgue, whose smiling, heartbreaking college pictures have been this week's headlines.

Her hands do not feel clean. She wants hot water and a scourer. She wants never to have touched that man, never to have had his glassy eyes and sweaty cold fingers on her arms as she told him lies while Sherlock crept up behind him with a piece of broken chair and a deadly expression.

The girls lay whimpering behind Joan and she faced down their kidnapper and would-be murderer, and Sherlock's face over the man's shoulder was hard and cold and frighteningly blank. Joan was quite prepared to attack the kidnapper, quite prepared to hurt him if it saved his victims' lives, but Sherlock's face showed more than horror at the terrible crimes. He was angry and afraid, and his face was frozen to try to hide it.

Joan gave Sherlock his chance, and the kidnapper fell. One blow, and then Sherlock had him in an armlock, and Joan ran to help the girls and to call, again, the police.

Now Sherlock has transferred his horror to fury and Joan is shivering.

"You ok?" Gregson asks her in a fatherly tone, dropping Captain duties for a moment and coming over. He has found a cup of coffee from somewhere - do Forensics have an espresso machine in the back of their truck? - and offers it to Joan, who shakes her head.

"I'll be all right." But she keeps winding the towel over and around her hands.

Sherlock is gesticulating at Detective Bell, his face crumpled into a frown, his coat creased and muddied from the struggle with the kidnapper, one knee of his jeans ripped. Unshaven and wild-eyed, his hair uncombed and his arms flailing with the passion of his words, he might be a street person urging passers-by to repent before the end of the world.

Joan hears him say, "These are obviously the tracks from a Dodge Charger with damage to the left inside tyre - the same car you dismissed from your case two days ago when the second girl was still alive!"

"Simmer down, Holmes, we got him," says Gregson, going over there.

"Not soon enough," says Sherlock furiously.

Bell retreats, leaving the Captain to handle Sherlock's wrath. He too comes and asks if Joan is ok, and again, she says she is. "You look a little shaken up," Bell observes.

"That man. The things he did. I've never dealt with anything like it before. That's all." The man was disturbed. Beyond disturbed. By the time Sherlock had him, flattening him with cold efficiency, the kidnapper was dribbling and spitting and his words were a stream of obscenities mixed in with the utterly mundane. He got taken away in a strait jacket.

Joan wants to go home. She wants her bed. She wants out of this place and away from the insanity.

"Yeah, good luck with that," remarks Bell, glancing at Sherlock who is cataloguing the nine key points of differentiation between the tyres on the killer's car and those of the car the police mistakenly tracked for two days, before they called Sherlock for help.

Joan whips round. "What do you mean?"

Bell steps back, holding up his hands. "Whoa, hey, I never meant - The stuff he knows, is all. All that stuff, that detail, he keeps in his head. Freaky."

Joan stares at Bell. She takes a couple of breaths but then it comes out anyway, and not quietly. "You don't think it's freaky that I have in my head all the stages of a hundred surgical procedures, do you? Or that guys in a bar have in their heads a thousand plays they can unpick throughout a football game? That's not weird, but Sherlock memorizing the results of a lifetime of close observation which he uses to help people, to solve hideous crimes, that is freaky, you think that makes him a freak?"

Her voice squeaks and Sherlock and Gregson turn towards her.

"I'm sorry," Bell says. "It came across wrong. I just meant that he's -"

"What?" Joan snaps.

"Special," says Bell, and Joan flips.

Sherlock reaches her side just as she is about to smack Bell across the chops, and moves swiftly to stand in front of her. "Watson. Watson! Our cab is here."

She is trembling, heaving in gulps of dank air, on the brink of assaulting a police officer, on the knife edge between rage and tears.

Sherlock hesitates, his gaze roving across her face, his own anger tamped down. Then he grasps Joan's arm and leads her quickly from the warehouse to where a taxi is parked with its engine running.

Joan allows him to chivvy her into the cab, and then sits, silent and resentful as they head back to Brooklyn.

"My word, Watson," Sherlock says in a light-hearted tone, "you are formidable when angry."

She frowns, sighs, shakes her head. Night-time New York flickers past the windows.

"And thank you," Sherlock adds in a fast mumble. He extends his hand to her, stopping short of contact - they do not touch except for practical purposes - then reels it back, drumming his fingers against his lower lip.

Joan's arm tingles with the touch which never materialised: Sherlock affection. Joan scowls, then flings herself back against the seat and shuts her eyes. She is exhausted.

Minutes pass and neither of them speaks.

"That man - the kidnapper - was released on a care order," Sherlock says after a while. "Negligence. The authorities completely overlooked the notes on his file which declared him a danger to others."

He gives her a sideways look. "I understand that it's hard to empathise. But this man actually begged the authorities not to let him go."

"So why did they?" At two a.m. and with the man's greasy eyes and grey skin still fresh in her mind, Joan is not in the mood for letting anyone off the hook.

"They needed the space."

"Huh."

"He was a very sick man and the people responsible for him let him down." Sherlock is firm.

"Let those girls down too."

"Yes. -You did an outstanding patch up job, Watson. The paramedics were almost superfluous."

"Huh."

"I do not praise to flatter you," he says then. "I only state facts. Your skill allowed me to focus on restraining the perpetrator."

"I'm just tired, Sherlock. Tired and strung out. I want to be in my bed."

"As do I."

She leans back in the seat and lets the lights flickering past the window mesmerize her. The city can be so bright and beautiful - yet it holds darkness too, and sometimes the darkest thing lies in the hearts of its people.

Eight million people. Even more lights. How can someone hide a secret like these evil, sick kidnaps? A sickness in plain sight, overlooked in the swirling mass of people and problems, in the sheer density of life in this crowded and noisy city.

Sherlock's gaze is on her, reading her thoughts in her face. Another of his skills, to know what she is feeling. Pity it doesn't cut both ways. How does he cope? Does he cope? "The kidnapper was sick, Watson. He was delusional. He thought he was doing one thing, whilst doing quite another. He will be convicted, but he will not go to prison. He will be sequestered for the rest of his life."

Joan recoils from the blurring city, from election billboards shouting Vote Latimer! and neon flashing the promise that such-a-store will supply your personal treasure. She turns her frustration on Sherlock. "How can we protect people against that? We can never be safe from those people."

She sees him flinch. Thinks of Bell and mentally kicks herself. But he only asks calmly, "What do you suggest, Watson? We should lock up everyone with any symptoms of mental illness?"

"No. Of course not. But-"

"There is no cure for it, Watson. There is only treatment. And care." His voice is low and gentle.

"It is hard to care for someone so - sick."

"Yes."

He turns his face from her then. The city blurs into sparkle and shine. Bright lights and black sickness are mingled together in the world's melting pot.

She cannot stop this kind of wrong. Sherlock cannot. It is invisible until too late. And she is so tired...

She is woken by the creak of the cab's brake, and Sherlock's hot hand rattling her knee. "Watson. We're home."

She stumbles into their brownstone, grateful for its warm, dim hallway, her head still full of bad dreams about crazy strangers and bright lights.

Sherlock pays the cab driver.

Joan is already at the top of the stairs when she hears Sherlock's soft, "Goodnight, Watson." She turns to see his mournful face tilted up to her in the brownstone's unique twilight, and his fingers fluttering an ironic farewell.

She opens her mouth to say goodnight, and maybe, come up, but he has already slipped into the library. She turns the corner and goes to bed alone.

Author's note: Bell is an idiot here but I put it down to the stress of the investigation and pressure from City Hall to clean up before NYC is a venue for the presidential campaign. He's just a little out of character for plot purposes!

Also, this story takes place after Declarations. It should stand alone, however. -Sef