A seven year old orphan boy is sent to live with his grandmother in Norway, where she tells him stories of witches who hate children, for children smell thoroughly vile to them. They have a number of distinguishing physical characteristics like baldness, lack of fingernails, pointed feet and so on, and they stop at nothing to eliminate all children, for they cannot stand them.
Through a series of events, they find themselves in a hotel in Bournemouth, which happens to be hosting the annual get-together of all English witches. Entirely by accident, the boy discovers their plan to convert all children to mice, involving a potion and a magic money-making machine.
The witches smell him and change him into a mouse, but aided by his grandmother, the mouse-boy steals the potion and feeds it to the witches instead. Together, using the money-making machine, grandmother and the mouse-boy decide to rid the world of all witches.
Although written for children, the book is tonally patchy and filled with plot-holes, deus ex machinas, and random coincidences which would have even the most gullible child reader rolling their eyes.