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On a stormy, gloomy night, Franklin Blake and Yandoo, his Hindu bodyguard and servant, arrive at the estate of distinguished scientist Sir John Verinder, whose daughter Anne is Franklin's fiancée. Franklin delivers to Anne a valuable diamond pendant, known as "The Moonstone," which years before had been stolen from an Indian temple. Also at the house are Godfrey Ablewhite, Anne's third cousin and book dealer; Septimus Lucker, a sinister moneylender to whom Sir John is in debt; housekeeper Betteredge; Cockney gardener Henry; Rosanna Spearman, a maid and former thief who is in love with Franklin; and Dr. Ezra Jennings, Sir John's assistant. That night, Anne puts the jewel, which enjoys a reputation for bringing its owner bad luck, under her pillow, then discovers it missing in the morning. Inspector Cuff of Scotland Yard arrives at the Verinder estate and questions everyone in the house about the theft. After Ablewhite and Lucker are allowed to leave, Rosanna is found dead in the garden, an apparent suicide. To protect Franklin, Rosanna, who had seen Franklin sleepwalk into Anne's bedroom in the middle of the night and steal the diamond, confesses to the crime in her suicide note and directs Franklin to look for the jewel under a stone in the yard. At the same time, Sir John is found unconscious in his bedroom, having collapsed from pneumonia contracted while venturing into the storm on a case. Dr. Jennings, with whom Sir John had been developing a secret formula, then informs Cuff that a delirious, dying Sir John had confessed to him that he had instructed Rosanna to place a potion in Franklin's milk to help him sleep just before the theft. To test Sir John's statement, Cuff orders a second dose of the potion to be given to Franklin that night and a phony pendant to be placed under Anne's pillow. As Cuff and the others watch, Franklin sleepwalks in Anne's room, takes a fake jewel and, in the hallway, mutters words that reveal that he previously had given the diamond to Ablewhite. At the pier, Lucker and Ablewhite are apprehended by police before they make their escape, while Franklin and Anne, reunited with "The Moonstone," embrace.