Abigail was thirteen. At the moment, it was nine o'clock, and her parents should have been back by eight. It was a Tuesday. Though it was getting past the point where their lateness could have been explained by a traffic jam or a shopping queue, she continued to fight down the panic.

Another of her headaches was coming on.

She'd always been prone to them, but over the past few months, they'd become stronger and more regular. They mostly ranged from a dull ache at the base of her skull to a fierce, throbbing pain directly behind her eyes, like a mallet thumping repeatedly at her brain. This one was the worst yet.

Trying to ignore it, Abigail fumbled in her bag with shaking fingers and extracted a sheet of Physics homework. She stared at it. The incomprehensible numbers and symbols seemed to spin, making her dizzy. Her head continued to pound.

It was now half-past nine. Outside, cars roared past, throwing up arcs of dark water, and far away she heard the wail of an ambulance – or was it in her own head? The sky overhead was a deep, fathomless black. No stars were twinkling. It was as if there was no sky at all.

It began to rain. In a daze, Abigail watched lines of water trickle down the glass, obscuring the view of the London night. The air in the flat smelt stale and musty.

At about ten, Abigail put a frozen pizza in the microwave, watching as the glowing green fluorescent numbers counted slowly and hypnotically down to zero, until the microwave dinged. Then she ate it. It tasted soggy and greasy.

At quarter to eleven, she called her parents. It was a long shot, as they rarely answered their phones, and as she'd expected the emotionless robotic voice answered, "The person you have called is not responded. Please leave your message after the tone."

She put down the phone. The rain fell harder and faster, blurring everything. Abigail felt as though she was the only thing that existed in the world. How was it possible to feel so alone?

Her headache seemed to be getting worse. Abigail put her hands over her face, relishing their coolness, and rested her elbows on the cheap plastic tablecloth. The momentary darkness was comforting, better than the stark yellow kitchen lights. Why weren't her parents here? What had happened to them?

At half-past eleven, she ran herself a bath. She put in too much bubble bath and the bubbles all ran over the side and went all over the floor.

Mum, Dad, where are you, where are you?

At twelve, she turned the lights out and put herself to bed.

She was woken early by the pain. Her headache had increased. In agony, Abigail began to rock slowly back and forwards. The bedroom was spinning like a merry-go-round. The flashing numbers on her digital clock read two-fifteen a.m.

Abigail went into her parents' room. The bed was clean and made, the covers tucked in. They hadn't been home.

Suddenly, she felt an overwhelming urge to look in the mirror, as though she needed to reassure herself that she was still the same person. She didn't feel like Abigail any longer. She didn't exist. Nothing existed.

Abigail went into the cold bathroom. Her own face looked back at her from the mirror. Pale, triangular, framed as usual by a long fall of mid-brown hair. And yet – something was wrong. Her normally white cheeks were flushed, and her hair was limp and greasy, hanging in rats' tails. Her eyes were too bright. They looked unnaturally large, wide and shining with an eerie glow, and they seemed too big for her face. Their colour was swallowed up in huge black pupils. Surely she didn't really look like that?

Abigail knew she ought to call the police. But something was stopping her. For some reason, it didn't seem like the right thing to do. She called her best friend, Gennifer, instead.

"Hi, Abi."

"Gen." Abigail's voice was frightened. In the dark, empty room, it sounded thin and tinny. "Something's happening…"

"What is it, Abi? What's wrong?"

"I…" Abigail put a hand to her forehead. It was cold in the flat, but she could feel sweat beading on her face. Her skin was hot to the touch.

"Gen…my parents…"

"Abi? Abi, are you there?"

Abigail dropped the phone. Gennifer's voice, distant and metallic, echoed out of it as it hit the ground. Abigail pressed the red button and disconnected the call.

What's happening to me?

Abigail was shivering all over. Sweat dripped off her. This wasn't normal. Everything felt weird and dreamlike. Dreamlike! Maybe that's it.

Maybe all this is a dream.

She pinched herself on the arm, and it hurt. She forced herself to concentrate on the pain, living it, breathing it.

What's happening to me?

She went into the kitchen, opened the cupboard and pulled out a block of cooking chocolate. She proceeded to eat the whole thing, breaking chunks off with her teeth, eating so fast she couldn't taste it.

The two deep crescents she'd made with her pinch began to bleed. Blood trickled off her arm and dripped on to the white kitchen surface. The red against the white looked oddly clinical, like something from ER. She breathed it in. It smelt salty and sharp.

Abigail tossed the wrapper aside and looked at her nails. They were bitten raw. How had she pinched herself so hard? I'm so hungry, she thought.

She raided the fridge and gulped down a hunk of cheddar cheese, and then a whole pot of creamy chive dip.

Then she ate a banana. Whole, with the skin and the little black nub at the top. Cramming it into her mouth.

Abigail went to the bathroom and put a plaster on her arm. It was still raining. Water ran down the window like the blood which had run down her arm, streaming clear, distorting everything.

Abigail stumbled and fell against the sharp edge of the kitchen surface. Yanking up her T-shirt, she saw across her ribs a cut edged with torn white skin, a line of red already swelling in the middle. She wiped her fingers across it. Licked off the red smears. She tasted them, and they tasted good.

That's better.

Abigail screamed.

She fought herself.

What's happening to me?

The thing masquerading as Abigail opened its eyes. Or rather, Abigail's eyes. But Abigail wasn't there anymore.

"That's better," she said. She went into the bathroom, just as Abigail had only a couple of hours earlier. She brushed her long hair until it gleamed gold.

I stared into the mirror. A pretty girl she had been, but oh, how messy! If she only took care of her body, she'd be so much more beautiful…

I went into her old parents' bedroom and ransacked the drawers. Scrabbling through them with my long fingers, I found a make-up bag and emptied it on the bed, spreading lipstick, eyeshadow and foundation across the dark blue checked duvet.

I applied them in front of the bedroom mirror, spreading the concealer, bronzer, blusher and foundation across my face – or what I now thought of as my face. I slicked on lipgloss, lip liner, lipstick, and Vaseline until my lips looked a little more defined. Much better. Then it was the eye makeup, shadow and liner and glitter and mascara, all different colours. Good, that looked much prettier now. Then I picked up some blue mascara and dragged it across my long brown-blonde hair, leaving streaks of sticky, clumped-together colour.

Hm. That was good, but it needed something more. Heading for the kitchen, I seized the tomato sauce and squeezed it over the strands, leaving long wet streaks. Oh, I looked like a real girl now! Nobody would be any the wiser.

Abigail smiled.

She blinked her eyes and shivered, like a blank-faced plastic doll.