The mirror had belonged to Nadia’s grandmother, and before that, to someone whose name no one in the family remembered. It was old — genuinely old, the kind of old that felt physical, like the glass itself was heavier than ordinary glass. The frame was dark wood carved with leaves and small figures that Nadia had never looked at closely because something about them made her not want to.

It arrived in a removal van on the seventh of October, the Tuesday after Nadia turned twelve. Her mother put it in Nadia’s room because it was the only room with a wall tall enough to hang it. Nadia stood in front of it for the first time and looked at herself — and then looked again, because something was not right.

Her reflection was wearing pyjamas. Nadia was wearing her school uniform.

She blinked. She stepped closer. The reflection did the same — but it was definitely wearing the pale blue pyjamas she wore on cold nights, the ones currently folded in her drawer.

The light, she told herself. Just the light.

She went to school. She came home. She ate dinner. She put on her pale blue pyjamas — and stood in front of the mirror, and her reflection was wearing her school uniform.

Nadia sat on the edge of her bed for a very long time.

She tested it the next day. She wore red on Wednesday. Her reflection wore blue. On Thursday she wore blue — and her reflection wore red. She held up a book and her reflection held up the book she would read on Friday. She pulled a face and her reflection pulled the face she would make on Saturday morning when her mother told her to tidy her room.

The mirror was showing her tomorrow.

For exactly nine days, Nadia kept this secret and used it carefully. She knew what the weather would be. She knew what her teacher would write on the board before class. She knew, on the Thursday, that her best friend Bex was going to fall off her bicycle — and she texted her not to ride it, and Bex didn’t, and nothing happened, and Nadia felt, for just a moment, like the most powerful person in the world.

Then came the night of the sixteenth.

She looked.

Tomorrow-Nadia was standing exactly where Nadia stood now, in the same pyjamas, in the same position. But tomorrow-Nadia’s hands were pressed flat against the glass — against the inside of the mirror — and her mouth was open in a shape that Nadia had never seen her own face make before.

It was not a scream. It was worse than a scream. It was the shape a mouth makes when it has been screaming for a very long time and no sound is coming out anymore.

Nadia stepped back. Tomorrow-Nadia pressed harder against the glass. Her hands left fog-prints that slowly faded — and where they faded, letters appeared, written backwards:

Nadia did not sleep. She sat on the far side of her bed with her back against the wall and her knees pulled up and did not look at the mirror. At three in the morning she put a jumper over it. At four in the morning she went and slept on the sofa downstairs.

In the morning her mother asked her what was wrong. Nadia said she had a bad dream. Her mother said bad dreams were just your brain sorting rubbish, which was comforting and also completely unhelpful.

Nadia spent all of Thursday finding reasons not to go into her room. She ate breakfast in the kitchen. She did homework at the dining table. She changed her clothes in the bathroom.

At eight in the evening, she heard something from her room.

A knock.

Not on the door. From inside.

She walked slowly up the stairs. She stood outside her room. The knock came again — three taps, slow and measured, from the direction of the covered mirror. She pushed the door open. The jumper was still over the mirror. The knock came once more — and she watched the jumper move, pressed outward from behind, as if something on the other side was leaning against the glass.

Nadia said, very quietly: “I know you’re not real.”

The jumper moved again. The shape behind it was not the shape of hands.

Nadia turned around and walked back downstairs and sat in the kitchen until her parents came home. She said, carefully and calmly, that she thought she should sleep in her parents’ room tonight, just because she had been having a hard week.

Her mother said of course. Her father said of course.

Neither of them went upstairs to Nadia’s room that evening. In the morning, when her father went up to get something from the airing cupboard, he called down: “Nadia — did you move the mirror?”

Nadia said she hadn’t.

Her father said: “Funny. It’s not in your room.”

They never found where it went. They looked through every room in the house, through the garden, through the garage. The mirror was simply gone.

That night, Nadia brushed her teeth in the bathroom and looked at the small square mirror above the sink — the ordinary one, the one that had always shown exactly the right thing at exactly the right time. She looked at her reflection. Her reflection looked back.

And then her reflection smiled.

Nadia had not smiled.

She turned off the bathroom light and did not look again.