John Herncastle, an English officer, kills three guards, and steals a precious jewel from the head of a Hindu moon-god, during the battle of Seringapatam. It passes into the hands of Miss Verinder, on the celebration of her 18th birthday. The Brahmins then come to regain it. That night the moonstone disappears; but who has it? The mystery is tackled by the resourceful detective, Sergeant Cuff.