In New York City's Central Park, Michael O'Hara, an Irish merchant sailor, rescues the beautiful Elsa Bannister from a group of thieves who are attempting to rob her. After fighting off the three attackers, Michael escorts Elsa to safety, and later learns that she is married to the renowned lawyer Arthur Bannister. The following day, Arthur, a cripple, offers Michael a job working on his yacht, and Michael reluctantly accepts the offer. The yacht sets sail from New York to San Francisco with Michael, the Bannisters, Arthur's partner, George Grisby, and a small crew. During the trip, Michael falls in love with Elsa, and learns that she agreed to marry Arthur only after he threatened to expose her shady past in Shanghai. Soon after the yacht reaches the Mexican coast, Michael and Elsa realize that they are being watched by the yacht's steward, Sidney Broome, who is actually a detective hired by Arthur to keep watch over Elsa. In Acapulco, Grisby tells Michael that he intended to disappear on the voyage so that his wife could collect his life insurance money. After explaining his plans to stage his own murder, Grisby offers Michael $5,000 to confess to killing him. Grisby assures Michael that he cannot be convicted of murder because there will be no corpse to prove his guilt. The voyage ends in San Francisco, where Michael, hoping that the $5,000 will buy Elsa's freedom from Arthur, agrees to sign a confession admitting that he murdered Grisby. Later, Grisby shoots Broome when Broome accuses him of planning the staged murder as part of a ploy to kill Arthur. The wounded Broome then finds Elsa and tells her that Arthur is about to be killed by Grisby. Meanwhile, Grisby and Michael enact the murder plot at the waterfront, where Michael fires three shots into the air, and Grisby speeds away on a motor boat. A short time later, Broome reports to Michael with his dying breath that Grisby arranged the phony murder to frame him for the murder of Arthur. Believing that Arthur has been murdered, Michael is surprised when he learns that Grisby has been killed. When the police find Michael's signed confession, they arrest him and charge him with the murder of Grisby. While visiting Michael in his jail cell, Elsa tells him that Arthur has volunteered to defend him. Though he distrusts Arthur, Michael is left with no alternative but to accept his help. Arthur deliberately presents a losing defense case, and, during the jury's deliberation, tells Michael that he hopes to send him to the gas chamber. When Arthur admits to Michael that it was he who killed Grisby, Michael creates a diversion in the courtroom by grabbing a handful of sedative pills and swallowing them. After punching Arthur and two bailiffs, Michael escapes and takes refuge in a Chinatown theater. Elsa finds him there, arranges to have her Chinese servant hide him, and promises to help him by looking for the gun that was used in the murder. While discussing the arrangement with Elsa, Michael accidentally discovers the missing gun hidden in Elsa's purse, and immediately realizes that Elsa is the one who murdered Grisby. When the pills he took begin to take effect, Michael loses consciousness and is taken by Elsa's servant to an amusement park fun house. Michael regains consciousness in a hall of mirrors, where he witnesses a gun duel between Elsa and Arthur. The Bannisters mortally wound each other in the duel, during which Arthur reveals that he sent a letter to the district attorney establishing Elsa's guilt. After Elsa and Arthur die, Michael leaves the amusement park, certain that the letter will exonerate him.