John and Margaret Begbie are picnicking with their parents in the Black Forest when they are mysteriously spirited back through time into the Middle Ages. There they meet Mardian, one-time servant and closest friend of Duke Otho, ruler of the country. He tells them a terrible secret - he is under the enchantment of the evil sorcerer Almeric, and is forced to assume a wolf's shape every night. Unable to return to the castle, the children are his only hope. And so John and Margaret disguise themselves as children of the court and embark on a thrilling and frightening quest to reveal Almeric's true nature and gain the Duke's confidence. But then Almeric becomes suspicious, and suddenly all their lives are in danger...