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At the Stamm family house, everyone waits for the arrival of Monty Montague, who is engaged to Bernice. Despite the fact that she and Dale Leland are in love, and that none of the other party guests, including Greeff, Tatum and her brother Rudolph have much affection for Monty, Bernice intends to marry him. Rudolph is intent on getting thoroughly drunk, so Leland offers to pick Monty up at the gas station where his car is stalled. He is about to leave when Monty and Ruby, another guest, arrive at the door. Bernice suggests that they all go for a swim. Monty and Leland dive into the dark pool, but Monty does not surface afterward. Frantic, Bernice has lights brought in, but a search fails to find him. The police are called and Sergeant Heath arrives, bringing District Attorney Markham and his friend, detective Philo Vance. Vance notices that Rudolph is a specialist in tropical fish. He questions old Mrs. Stamm, a harmless lunatic, who insists that a water monster killed Monty. The next day they drain the pool but do not find a body. Vance spots three-toed tracks in the mud at the bottom and later finds the body in a pot hole. Dr. Doremus, the coroner, remarks that there is no water in the corpse's lungs and notes strange marks on its throat and chest. Then Trainor, the butler, is knocked unconscious with a blunt instrument and Greeff disappears. Vance again questions Mrs. Stamm, believing that she is not as crazy as she appears. He discovers that the key to the family burial vault is missing, and after a search, it is found in Tatum's bed. Tatum, however, denies any knowledge of the key. Vance gathers everyone at the pool to reenact the events of the murder. Rudolph plays the part of Monty and after his dive, fails to surface. Vance tells everyone that Monty was murdered by someone wearing a diving suit, which is what made the strange marks. They search for diving equipment in the vault and discover Greeff's body in one of the coffins and the missing equipment in another. Vance explains that Rudolph was only pretending to be drunk the night of the murder. He owed Monty money and Monty was using the debt to force Bernice to marry him. Greeff was killed because he knew too much. Now Leland and Bernice are free to marry.