She’d planned it for months. Hours in her local library with encyclopaedias and research books, explaining to the staff that she was looking for information for a school project. They’d looked at her with indulgent smiles and told her to help herself. Not once did they ever investigate what the nine-year-old was reading. Just as well as no school project would have involved the best way to kill someone using whatever was available. She was searching for a simple, painless way for her parents to die, to make way for the adoptive parents who would adore her.

It was, in the end, her parents themselves who gave her the idea.

‘I was so cold last night,’ her mother complained to her father one morning. ‘We should get a heater for our bedroom.’ Their bedroom. Not Allison’s room or those of her siblings.

‘There’s that old kerosene heater we used to use when we went camping. It’s out in the garage rusting away; why don’t you bring that in?’

Allison remembered her mother laughing and thumping her father’s arm. ‘Trying to kill me, are you? Don’t be daft!’

That afternoon in the library, Allison read everything she could find on kerosene heaters. She took notes, covering page after page in her copybook with her neat writing, checking and double-checking that she had it right. When she returned home, she dropped her schoolbag on the floor and headed out to the garage.

A graveyard for redundant and broken household items… the electric mixer her mother had used once and decided she hated; a TV with a smashed screen; an old chair with three legs. Allison had to rummage through mountains of rubbish, brushing aside cobwebs and their leggy occupants with a grimace, focused and determined to find the heater.

And there it was, hidden beneath a moth-eaten rug. She picked it up and examined it. It looked similar to ones she’d seen in the library books. Putting it on the floor, she tried the ignition. It was very old; it probably wouldn’t work. But it did, first time. She looked at the flame with a growing smile, then switched it off. It was perfect.

There was no point in wasting time. With a look outside to make sure her mother was out of sight, she took the heater into the house and hurried up to her bedroom. It fitted neatly inside her small wardrobe.

Her plan was simple. Both her parents were heavy sleepers. Allison would stay awake, wait till an hour after they went to bed, then she’d turn the heater on, put it in their bedroom, and shut the door. A rolled-up towel at the bottom of the door would make sure all the carbon monoxide given off by the heater would stay inside.