In her own cottage a few miles away the witch Agnes Nitt was in two minds about her new pointy hat. Agnes was generally in two minds about anything.

As she tucked in her hair and observed herself critically in the mirror she sang a song. She sang in harmony. Not, of course, with her reflection in the glass, because that kind of heroine will sooner or later end up singing a duet with Mr Blue Bird and other forest creatures and then there's nothing for it but a flamethrower.

She simply sang in harmony with herself. Unless she concentrated it was happening more and more these days. Perdita had rather a reedy voice, but she insisted on joining in.

Those who are inclined to casual cruelty say that inside a fat girl is a thin girl and a lot of chocolate. Agnes's thin girl was Perdita.

She wasn't sure how she'd acquired the invisible passenger. Her mother had told her that when she was small she'd been in the habit of blaming accidents and mysteries, such as the disappearance of a bowl of cream or the breaking of a prized jug, on 'the other little girl'.

Only now did she realize that indulging this sort of thing wasn't a good idea when, despite yourself, you've got a bit of natural witchcraft in your blood. The imaginary friend had simply grown up and had never gone away and had turned out to be a pain.

Agnes disliked Perdita, who was vain, selfish and vicious, and Perdita hated going around inside Agnes, whom she regarded as a fat, pathetic, weak-willed blob that people would walk all over were she not so steep.

Agnes told herself she'd simply invented the name Perdita as some convenient label for all those thoughts and desires she knew she shouldn't have, as a name for that troublesome little commentator that lives on everyone's shoulder and sneers. But sometimes she thought Perdita had created Agnes for something to pummel.