Rose Rita Pottinger is dreading summer. With her best friend, Lewis Barnavelt, away at Boy Scout camp, vacation threatens to be altogether boring. But when Mrs. Zimmermann, Lewis's next door neighbor and a genuine witch, receives a strange deathbed letter from an eccentric uncle, unexpected things start to happen.

Rose Rita and Mrs. Zimmermann set off on a trip to discover the meaning of the letter. A ransacked farmhouse, a missing ring, shadowy figures appearing in the night, and mysterious magic symbols are just the beginning as they are gradually drawn into a terrifying world of occult mysteries, where Mrs. Zimmermann's failing powers can't help them.