The doorbell rang. Joan set aside the map pinpointing drug raids across the city, and went to answer. Sherlock had been barricaded in the back room with a mystery visitor all morning, and there was no chance of him getting the door.

A cab driver stood on the doorstep. "Car for Mr Kay."

"Right."

Joan went through to the back room. She had been hearing animated discussion from behind the locked door all morning: Holmes' dry, steady proclamations, and his visitor's higher pitched, urgent sentences. The words were muffled, but Joan knew it must be about the influx of drugs into the city which had led to a surge in overdoses and drug related violence.

Joan knocked. No reply. She opened the door.

No sign of Sherlock. His guest was hunched at the table nursing a small flat bottle of whisky.

"Cab for Mr Kay," said Joan uncertainly.

Kay stood, wobbling a little, and pocketed the drink with filthy, trembling hands. He had a terrible burn or birth mark across his face; his left eye was almost closed and his mouth was drawn downwards on that side by the red puckered flesh. He stumbled past her without a word, but his stench was enough to make her recoil and press her hand over her mouth.

"Where's Sherlock?" she called after him.

"He went out," in a thick Bronx accent, was all the reply she got.

Great. Joan opened the window to let the stale air out. She heard the cab roar away, then saw a scrap of paper on the table where Kay had been sitting.

It was in Holmes' tight, rapid scrawl. "Haitian link to drugs. Possible use of homeless as me outside the Unified Mission by Riverside Park to investigate. S."

Joan sighed.

On the subway she picked up an abandoned newspaper. The headline was typically sensational: Mutant Killer Stalks City's Darkest Places.

Joan rolled her eyes. The piece was about a mysterious deformed man, believed to be a Vietnam veteran, now living in the disused tunnels under New York. The man had supposedly been suffering some kind of mental trauma which had led him to begin killing his fellow subterranean dwellers.

Joan set aside the paper as the subway rolled in. New York was full of such myths, the more extreme, the better.

The Mission occupied a drab concrete building opposite the park. A few bleak fluorescent lights were on inside, showing a bare office room and a functional canteen where people sat drinking coffe or eating soup.

Joan shivered outside until a young woman with no make-up, in a minister's garb came down the steps to greet her. "You must be Joan Watson," she said. "I'm Clare Webb."

"I'm supposed to meet Sherlock here," Joan said, looking around.

Clare looked thrilled. "That would be amazing. We've spoken on the phone, but never met. I work with the homeless community here, and Sherlock consults me about what's happening with the dispossessed."

"Right. Has he been talking to you about drug running being carried out by homeless people?" Joan asked.

"No," said Clare, "but one of my regulars said something about the people going missing in the tunnels, so I gave Sherlock a call." She glanced around. "Oh, there he is. Let me go have a word. If you don't mind, best you stay over here, these people don't always connect so well with strangers."

Joan looked across the street and recognised Kay, the man who had been at the brownstone earlier.

Clare spoke to him. He pointed across the park. Clare seemed troubled, and after more chat to Kay, gestured to Joan to join them. But as she crossed the street, Kay shuffled off into the park, disappearing behind the trees.

Clare sighed."That's Kay," she said. "He drops in from time to time, usually when something bad has happened. He's been around a lot more lately since these tunnel killings."

"Are those real?" Joan asked. "I mean, is there actually a deformed crazy person killing off homeless people in the subway system?"

Clare grimaced. "Well, it's hard to verify. People are scared, that's for sure."

"What did Kay say to you just now?"

"He said the killer is stalking the old Amtrak tunnel under Riverside." Clare took a breath. "And that anyone who wants to live, had better stay away."