A walk in Wolf Wood - Mary Stewart

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 . . .