"Ever had any security issues?"

"Kids sometimes break in looking for cash in the register or raiding the shop. The coppers said something about a s.e.xual a.s.sault. Never had something like that. They asked about CCTV cameras. Can't have cameras in changing rooms. Imagine the problems. Privacy issues."

The male and female changing rooms are in opposite corners. Opening a control panel, he flicks more switches and unlocks the doors. Rubber non-slip mats form a rippled path between the pool and the showers. Rows of lockers are arranged into alcoves with wooden benches in between.

I recognize the benches. This is where they sat as Natasha danced. Seven men set out to punish her. They talked of cutting her hair or scarring her face. Branding her. This is the power of people combining, when individual responsibility is diminished and the mob holds sway. It happened on the night that Augie Shaw died. It happened during the summer riots in London.

Regardless of the hows and whys, the psychology remains the same. The crowd provides anonymity, it abrogates responsibility, it diminishes any sense of "self." People don't lose their ident.i.ty-they gain a new one. They unite against a common enemy, perceived or otherwise. They become a tribe.

Seven men imprisoned and a.s.saulted a teenage girl. I don't know if they raped her. Collectively they justified something that individually they would never contemplate or carry out. They whipped her, making her dance, treating her like a performing animal instead of a human being.

Yet under different circ.u.mstances, if I had met some of these men on their own, I might have seen decent, hard-working, law-abiding human beings; men who love their kids, are loyal to their wives and are kind to animals. I'm not trying to excuse their behavior; I'm trying to explain it.

The a.s.sault footage was time-coded. The camera stopped at 11:17 p.m. Piper Hadley showed up at Emily's house just before midnight. She could have gone to the police, but she didn't. Maybe she was scared. The last time Natasha was a witness at a trial, she was treated like the accused and Piper was sent away to a school for troubled teenagers.

The girls didn't spend the night at Natasha's house and Mrs. McBain didn't wake them in the morning. So where did they go? When were they taken? Most likely the kidnapper was one of the men who imprisoned and a.s.saulted them earlier in the night. He came back afterwards, or intercepted the girls. Chose the right moment.

Mr. Creighton is growing impatient. I follow him along the pool deck and out through a side door, where dead leaves have piled up against the fences, marshaled by the wind. The air is cold and bright, smelling of wood smoke and wet clay.

My mobile vibrates against my heart. There is a text message from Dr. Leece at the hospital: he wants to see me.

The vertical blinds are drawn in the pathologist's office. I knock. A voice tells me to enter. John Leece is sitting in semi-darkness, reclined on a leather office chair with a wet flannel over his eyes. A lone desk lamp throws a circle of light over a series of post-mortem photographs arranged on his desk.

"A migraine," he explains, without removing the flannel. "I've been plagued by them since childhood."

He motions me to sit down.

"There are times when I'd gladly put a bullet in my head to be rid of the pain."

"Quicker than aspirin," I say.

"Rather more permanent."

He folds the flannel and dips his fingers in a gla.s.s of water before wiping them across his eyelids. I'm studying the photographs on his desk. One of them shows Natasha's body beneath the ice, her face in soft focus. A shaft of sunlight has pierced the clouds and illuminated the snow, creating a halo around her head. She looks almost peaceful, as though she's been laid to rest in a mausoleum of ice.

"You wanted to see me."

"Have you heard of tritium?"

"No."

"It's a hydrogen atom that has two neutrons in the nucleus and a single proton. Although it can be a gas, it most commonly reacts with HO to form tritiated water. Colorless. Odorless. Although not particularly dangerous, it's considered to be a low-level radioactive hazard with a half-life of 12.3 years. The molecule is so small it enters the body very easily-inhalation, ingestion, even through the skin."

"Why are you telling me this?"

"Traces of tritium showed up in Natasha's urine. At some point she ingested the molecules, probably in contaminated water. Maybe she bathed in the water or was given it to drink."

"Radioactive water?"

"Normally it would be excreted through her urine within about thirty days of ingestion, so she must have been contaminated in the past month."

"Is it dangerous?"

"Tritium is created naturally in the upper atmosphere when cosmic rays strike air molecules, but it's also produced during nuclear weapons explosions or as a by-product of nuclear reactors. It's a pollutant that in large enough doses can cause cancers, congenital malformations and genetic mutations."

"Where was she contaminated? Radley Lakes?"

"It's a wildlife sanctuary. They test the water regularly. Contamination at these levels would have showed up." He gets to his feet and crosses the office to his desk. "I talked to someone from the Nuclear Decommissioning Authority. He didn't believe me at first. He said tritium exposure is almost unknown but that occasionally there had been accidental releases into the cooling water of nuclear power plants or into the effluent stream as waste."

"So what you're saying is that Natasha was exposed to some form of nuclear waste or spillage?"

"Yes."

"From a nuclear power plant?"

"That's the most likely source."

"Didcot Power Station?"

"Didcot is coal and oil," says Leece.

"Where then?"

"I was thinking about Harwell. It's only sixteen miles south of here."

"I thought it was decommissioned years ago," I say.

"The last three reactors were closed in 1990 and the land was remediated, but there are still storage sites with radioactive material. It's going to take another ten years to clean them up."

Dr. Leece opens his laptop and calls up the results of an Internet search. Harwell was Britain's first nuclear reactor, set up in the 1940s when the government handed over an RAF base to the Atomic Energy Research Establishment.

"Treated waste water from the old nuclear plant was piped to the Thames at a place called Sutton Courtenay, which is only a couple of miles south of Radley Lakes."

He calls up Google Earth. A picture of the planet appears on screen, as though taken from outer s.p.a.ce. The camera plunges towards the surface, falling onto Oxfordshire. Slowing. Stopping. Focusing.

"There's also this place," says Dr. Leece. "The Culham Science Center is a research laboratory experimenting with nuclear fusion as part of the Joint European Torus project."

"Where is it?"

"About a mile south of Radley Lakes."

"So the tritium could have come from there?"

"I'm just suggesting it's a possibility."

I look at the screen. The main railway line from Oxford to Didcot runs past the research center. Natasha McBain could have followed the tracks in the blizzard, trying to get home.

"Does DCI Drury know this?"

"I left him a message."

"This is where they should be searching."

"They won't look now-not until after Christmas."

I wake shivering.

I don't know how long I've slept, but I can't feel my feet or my toes. The blood has dried on my knees, but the scabs split when I bend and the wounds begin weeping again.

George said it was Christmas Eve. I dreamt that I could see my family around the table: Dad, Mum, Phoebe, Ben, Granddad and the little sister I haven't met.

I slide on my stomach to the edge of the overhanging rock. The ground is wet and it seems colder now, cold enough to snow.

Climbing out of the crevice, I peer over the top of the rocks, studying the path. I can't see George. Can't hear him. The trees are like charcoal drawings above me.

I pick up the coat and brush off the leaves, putting my arms through the sleeves. It's too long for me. I fold up the sleeves and put my hands deep in the pockets. It smells of George.

My fingers touch his mobile phone. I'm so surprised that I almost drop it. Two hands. I turn it over. Look for the power b.u.t.ton. The screen lights up and welcomes me with music. There are no bars of signal. If I get higher, I might pick up a phone mast.

Scrambling up the embankment, I keep tripping over the hem of the coat. I have to hike it up and hold it under my arms, which makes it hard to climb because I can't hold onto the trees.

When I reach the path, I crouch behind a rock, peering both ways. I can't see him. I don't want to go back the way I've come, so I keep following the path, away from the factory, looking for a road or a house or a car.

It's raining and misty, but I can make out a trail that snakes between the trees. I'm climbing. That's good. Maybe I'll get a signal from higher ground.

Every few minutes, I stop and look at the phone, checking the signal. One bar blinks for a moment and then disappears. I wait. It flashes again. I scramble onto a rock and hold the phone above my head. A second bar of signal appears alongside the first. Wider. Stronger.

I dial 999. An operator answers.

"h.e.l.lo, what service do you require, police, fire or ambulance?"

"Police. I need help."

"Can I have your name please?"

"I'm Piper. He's chasing me, please hurry."

"Hold the line."

A different voice answers this time. A woman.

"You're through to the police. Can I have your name please?"

"I need you to come and get me. He's going to kill me."

"Please, tell me your name?"

"Piper Hadley."

"Has there been an accident, Piper?"

"No. He's coming, please help."

"Who is coming?"

"I don't know his name. This is his phone."

"Where are you, Piper?"

"In a forest."

"Whereabouts?"

"I don't know."

"So you've just wandered into nowhere?"

"I was kidnapped. I've managed to get away. You have to come quickly. He's got Tash. I know he'll punish her."

"Who is Tash?"

"She's my friend. We were kidnapped together."

"What's your friend's name?"

"Natasha McBain."

"You're breaking up, Piper. Can you please repeat the name?"

"I said Natasha McBain."

"Is this a hoax call?"

"What?"

"Do you know the penalty for making false emergency calls?"

"It's not a hoax! It's not!"

"There's no need to yell, Piper. If you become abusive, I will terminate the call."

"I'm not being abusive. I'm telling you the truth."

"I'm going to need a better location. I need a street or a cross-street."

"There are no streets."

"I didn't catch the name of the street."