Late 1967, a young boy, moves in with his grandmother after he is left an orphan. The boy and his grandma share as special connection; she tells him many stories about witches, so that he could recognise them and keep safe. Apart from a few distinguishing physical characteristics (they are bald, have clawed hands, and lack toes; mentioned in the extract), witches look like human women, but are awful creatures that prey on children. The Grand High Witch, the most-evil and most dreaded leader is the main witch his grandmother cautions him about. On the English coast, the grandma and the boy go on holiday; which is where the boy has his encounter with a group of witches gathering for their yearly meeting.

As he eavesdrops, the grand witch introduces her new invention she calls “Formula 86”. This is a potion to turn all children into mice, the witch explains putting the potion into candy whilst having witches open all sweet shops. The Grand High Witch lures a boy named Bruno Jenkins, as part of her presentation and turns him into a mouse. Soon after, she discovers the boy and turns him into a mouse as well.

Despite being transformed, the boy finds Bruno and realises that both of them still regain their voices, intelligent and personality. The boy strategies a plan to turn all the witches into mice, by stealing the same potion and placing it into their food. The plan succeeds, with all the witches transformed into mice and slaughtered by the hotel staff and guests. When returning home, the boy plots revenge with his grandmother to use the potion to eradicate all the witches in the world. Despite still being a mouse and only having another nine years to live, he is happy to live as long as his grandmother is beside him.