Marianne and Joe Pinhoe, youths in a clan of rogue magic users, are looking forward to their summer holiday, only to have it ruined by Gammer Pinhoe, the scheming, elderly matriarch of the family. Joe is sent to work at Chrestomanci castle to spy on the wizard folk there, while Marianne must look after Gammer.
But when a visiting clan leader comes to speak to Gammer, the ensuing argument so enrages Gammer that her mental capacities become impaired and a magical war breaks out between the two clans. Sure that a spell is responsible for Gammer’s insanity, Marianne tries to convince her family but nobody believes her, and when she finds an enormous egg hidden in the attic, things go from bad to worse. Marianne goes to Joe, but finds Cat, the young nine-lived enchanter who lives at the castle, instead. With his, Chrsetimanci’s and Joe’s help, Marianne must find a way to stop the war before anyone gets seriously hurt.