A shrieking, killing nightmare erupts from an overgrown well, hidden in the grounds of an old house, Tranchard's Folly — and Mary Shelley, the Doctor's latest travelling companion, rescues teenage twins Finicia and Lucern from the clutches of the monster. But a TARDIS trip in search of the origin of the horror goes terribly wrong when the Doctor, Mary and their two new friends find themselves stuck in the middle of a seventeenth century witch scare.