The moonstone in the title refers to a brilliant but flawed gem seized by a British officer in India. He brought it back to England as a family heirloom - with a supposed curse placed upon it. The officer bequeathed the stone to his niece, Rachel Verinder, for her to inherit when she turns 18. The night of her 18th birthday, the Moonstone goes missing. Everyone connected with Rachel at her family estate in Yorkshire is under suspicion. It is up to the London detective, Sergeant Cuff, to solve the crime.