Abingdon Police Station never sleeps. Shifts change. Fresh faces replace tired ones. Doors swing open and close behind us. Drury ignores greetings or dismisses them. Reaching the incident room, he throws his coat over a chair and yells to the a.s.sembled detectives. A briefing. Fifteen minutes.

I'm to wait in his office. Not touch anything. There is a whiteboard with photographs of the farmhouse and the victims. Natasha McBain's image has been placed off-center, as though peripheral to the main investigation, yet now she is at the heart of it.

Taking a seat, I glance around the room. A cupboard door is open. There are press clippings stuck on the inside of the door, a bravery citation, photographs of a medal ceremony. Drury is bowing to the Queen. The office door opens before I can read the caption. The DCI is carrying two mugs of tea, presenting one to me like it's a peace offering.

He takes a seat behind his desk.

"OK, let's a.s.sume you're right and Natasha was at the farmhouse that night. What happened?"

"She arrived during the blizzard. Wet. Cold. They drew her a bath. Found her fresh clothes. Dried her shoes in front of the fire. William Heyman tried to phone the police, but the switchboard was overwhelmed."

"And then Augie Shaw showed up?"

"Someone did."

A church bell is ringing somewhere. Drury scratches the short hair on the back of his neck.

"Half my team worked on the original investigation."

"Is that a problem?"

"Decisions were made based on the best available evidence. The girls were cla.s.sed as runaways. When they didn't turn up after three months the chief constable a.s.signed a smaller task force but the trail had gone cold. Questions are going to be asked. Fingers pointed. This could cost people their careers."

"Is that your priority?"

The DCI bristles, opening his mouth and closing it again, his lips like thin lines.

"I'm not here to judge anyone," I say. "I'm reviewing the evidence not the investigation."

Drury grunts, unconvinced.

The landscape has changed since the girls went missing. The science has improved. Offenders will have grown complacent. People will have forgotten their motive for lying. Lovers give alibis but ex-lovers take them back. I could make these arguments, but I doubt if Drury will listen. He's protecting his patch and the reputations of his colleagues.

"I'll probably regret this, Professor, but I'm going to give you full access. Don't turn this into a witch hunt. What do you need?"

"I want to visit the farmhouse again."

"Fine."

"I'll need the files from the original investigation," I say. "Statements. Timelines. Phone wheels."

"You're talking about more than three thousand statements."

"I'll get some help."

Drury swallows something spiky and hard. "Tell Grievous what you need."

"I also want to re-interview some of the original witnesses. Talk to the families, filter out some of the biases."

"You think they lied?"

"People edit out the negatives when they lose someone they love. You heard Alice McBain talk about her daughter. I need to learn everything I can about Natasha and Piper. What sort of girls were they? Were they streetwise, or naive, aggressive or compliant, introverts or extroverts? Did they have boyfriends or ex-boyfriends? Were they s.e.xually promiscuous?"

"You're suggesting these girls were somehow complicit?"

"I'm saying that some women-even young ones-draw attention to themselves. Some are s.e.xually provocative, deliberately or unwittingly. Others are more self-effacing. I need to know Natasha and Piper as though they were sitting opposite me. By knowing them, I can learn why they were chosen-"

"You think they were chosen?"

"Yes."

Drury breathes deeply, loosening his shoulders, staring at me.

"I've met people like you before, Professor. You study crime scenes and photographs, thinking you can commune with the killer; trying to understand the whys and the wherefores. Me? I don't care about knowing the b.a.s.t.a.r.d. I just want to catch him."

Two dozen detectives are gathered in a rough circle, sitting on desks and chairs. They share the same kind of intimacy as soldiers and emergency workers-friendships forged in the heat of battle or during long shifts doing dirty and dangerous work. Not elitist or self-anointed, just tight.

Drury calls for their attention.

"Listen up, lads. Some of you may have heard a rumor about the unidentified white female whose body was found after the blizzard. We now have a positive ID. Her name was Natasha McBain."

The air pressure in the room has suddenly changed, as though someone has opened a distant door letting a cold wind blow through the corridors.

"I am now going to confirm something else," continues Drury. "The rumors stop now! n.o.body-and I repeat, n.o.body-talks to the media. I'm declaring a total news blackout on this case. Whatever you get asked, the answer is 'no comment.' I don't care if it's your wife asking the question, you say nothing. Is that understood?"

n.o.body interrupts.

"I want to know where Natasha McBain has been for the last three years. Go back over the files. Names. Dates. Places. I want a full list of suspects from the first investigation. Where are they now? What have they been doing?

"We're going to search the crime scenes again-the farmhouse and the lakes. Uniformed officers and civilian volunteers are being bused in within the hour. The dog squad will try to pick up the scent using Natasha's clothes. n.o.body mentions her name. As far as anyone is concerned we're still dealing with an unidentified white female."

A voice from the back: "What about Augie Shaw?"

"He's going nowhere. Find out if he knew Natasha McBain or Piper Hadley."

"And the Heymans?"

"Victims in one crime, suspects in another-it's not the first time." The DCI looks at Casey. "What about the prints at the farmhouse?"

"We pulled sixty decent samples from the house and have eliminated all but fourteen of them."

"Augie Shaw?"

"A palm print in the kitchen."

"Anything upstairs."

"There's a partial on the bedroom door."

"Find out who else has been in the house in the past month. Tradesmen. Friends. Family. What about the s.e.m.e.n stains?"

"DNA results will take another two days. The daughter says her parents had separate bedrooms and weren't sleeping together."

"Kids don't know everything. Maybe they were loved up behind her back."

Another detective speaks. "There were traces of diluted blood on the broken bathroom window and in the kitchen sink. We'll have to wait for the results."

Drury looks at another detective. "What about the family finances?"

"A mortgage. Manageable."

"Good." Drury slaps a folder against his thigh. "At the behest of the chief constable, we are to welcome back Professor O'Loughlin. He is to be afforded every reasonable a.s.sistance but don't get carried away with his theories. We're going to solve this case through good, solid detective work, by knocking on doors and interviewing witnesses."

Point made, Drury doesn't look at me.

"I'm splitting the task force. DS Casey will continue to run the investigation into the double murder at the farmhouse. I'll be in charge of the Natasha McBain investigation, but overseeing both."

He rattles off names, a.s.signing detectives their new roles.

"Let's do this," he says, turning and leaving quickly, only letting his mask slip when he reaches the corridor. I see the glaze of uncertainty dulling his eyes like Vaseline smeared on a lens.

Sometimes I wonder why detectives do this work. What pleasure is there in it? Even the satisfaction of solving a case just means another one is waiting. There is never a cessation of hostilities or a negotiated truce, never ultimate victory.

Eventually, the eternal nature of the struggle wears them down-the circle of cause and effect, crime and punishment, guilt and innocence, victims and perpetrators. You don't stop feeling-you just wish you could.

I was born on Mother's Day and Mum used to say I was the best Mother's Day present in the world. She said things like that when other people could hear her, but never when it was just me listening.

We didn't talk. We competed. We argued. We loved each other. But we hated each other too.

My mother was the world champion at making smiley comments about my hair or my weight or my bra size, slicing and dicing my self-confidence. And she was never happier than when dancing through the tulip fields of the bleeding obvious.

Dad would tell me not to get so bent out of shape, but I was born bent out of shape. I came into the world backwards in a breech birth. Whales breach and so do babies.

Mum is taller than my dad but really skinny. She has these amazing green eyes and eyelashes that look like they're false but they're not.

People say she's beautiful and talk about Dad "punching above his weight" when he married her, but I think he could have done much better. He could have married someone who didn't care so much about money and what other people thought.

My dad is the nicest person you'll ever meet. Whenever he's disappointed in me he has this way of sagging and letting out a long sigh, as if someone has pulled out his plug and he's crumpling like a bouncy castle at the end of a party. He would die of disappointment rather than raise a finger against me.

Mum used to complain when he spoiled me and Dad always agreed with her before winking at me.

My last birthday at home was cancelled because Mum said I didn't deserve a party or presents because of my ingrat.i.tude and my filthy language, particularly the word "f.u.c.k." Everything was f.u.c.king this and f.u.c.king that; f.u.c.king unfair and f.u.c.king unbelievable and you have to be f.u.c.king kidding me.

That's one of the reasons I wanted to run away, but it was just talk, you know. I wasn't really serious. Kids always say things they don't mean.

It's morning. I stand on the bench and see if the sun is shining or if it has snowed overnight. No snow. No sunshine. Rain today. It's colder than yesterday.

Standing here, I can almost feel the weight of Tash kneeling on my shoulders and then standing, as she squeezed through that narrow gap. I was afraid that she'd get stuck and I wouldn't be able to pull her back inside. She'd be like Winnie the Pooh in that story where he eats too much honey and gets stuck in Rabbit's front door.

I wet my finger and hold it against the gap, feeling the breeze on my skin. Then I draw a heart in the condensation on the inside of the window. Why do people always draw hearts?

It's been four days since Tash left. That might not seem to be very long after three years, but some days are longer than others. Some days are longer than years.

Only one of us could escape because we couldn't both climb that high. One of us had to lift the other. Tash was smaller. She'd lost so much weight.

Ever since George made her bleed, Tash had been acting differently. I don't know if she tried to stab him with the screwdriver. She wouldn't talk to me. Instead, she scratched at her wrists, biting her nails, sleeping all the time... I tried to talk to her... to make her eat, but she didn't even have the energy to argue with me.

"You're scaring me," I said, rocking her in my arms. "Please come back."

"We're going to die," she whispered.

I knew she was right. It was like a message from G.o.d. A pretty disappointing message, but I didn't blame him. That's what everything comes down to-dying. Well, not literally everything, but most things.

Tash didn't seem scared any more. Perhaps knowing you want to die makes you less scared. Sometimes there's no rock so heavy or dark or hopeless that people won't crawl under it.

The idea came to me when I was standing like this, looking through the crack. I noticed how the condensation on the inside of the gla.s.s had leaked down and frozen along the bottom edge of the window. The ice had expanded in the crack and forced the metal frame to lift. I could see a c.h.i.n.k of light where there hadn't been one before. My old science teacher taught me that water expands when it freezes. That's why it can break open granite boulders.

I thought, If it could break a boulder why not a window or a wall?

So I filled a bowl with water and tore up an old T-shirt. I soaked the torn fabric and shoved it into the gap, using a nail file to push it hard into the s.p.a.ce. Some of the water squeezed out and leaked down the wall.

It was cold that night. The fabric froze. The next day, I pulled it out and wet it again. Night after night, it froze and refroze. For a long time I didn't think it was working. The gap looked the same. But then one day, I pushed at the window and the whole thing moved.

Some nights weren't cold enough for the fabric to freeze, but then we had a long cold spell. We shivered and huddled together at night, trying to stay warm. And each morning the crack had opened a little more.

I wedged my fingers inside and to my surprise the window moved. I tried again and it gave way. I caught it before it crashed to the ground, falling backwards off the bench. The edge of the window frame cut my forehead, but it wasn't so deep.

Where the window had been, there was now a hole. Tash couldn't fit through it, so she took most of her clothes off. First she knelt on my shoulders and then she stood. Once she put her head and arms through the window, I pushed and she clawed at the ground, trying to pull herself through.

She wouldn't budge. I couldn't pull her back or push her forward. That's when I got really scared. I thought she was going freeze to death stuck in that window, lying half in the snow. I managed to pull off her leggings and then I poured vegetable oil over her hips and thighs.

"I can't do it," she kept saying.

"Sure you can."

"I can't."

"Wiggle your hips."

"I'm stuck."

"Keep trying."

"I am trying."

She was swearing at me and crying. I had to scream at her and slap her on the thighs. I hit her so hard she slipped right through that hole, her legs and feet slithering out of sight. Snowflakes drifted in. Her head reappeared. I grabbed some more clothes and pushed them through the window.

"I'll be back," she said, all business. "Don't go anywhere."