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Convinced that glandular disorders are at the seat of all criminality, Dr. Herbert Stander begins experimenting with endocrine injections. To conduct his experiments without interference, the doctor charters a yacht, which he convinces his nephew, Lieutenant Bob Bennett, to captain, then helps eight killers escape from jail and brings them aboard as guinea pigs. On board, a fight erupts when Mary Slavish, a passenger of the boat, accuses her secretary, Joan Martel, of turning her into the police for murder. Joan, who was unaware that her boss was poisoning people to collect on their insurance policies, falls in love with Bob, who asks his uncle for help in proving her innocence. After one of his subjects dies, the doctor decides that he must experiment on a normal person and designates Bob as his next subject. Joan grabs a gun and tries to save Bob, and in the confusion, he switches the solution in his injection to water. When the doctor plans to use Joan as his next subject, Bob feigns confusion from the injection, steals a gun, frees the killers, and organizes a mutiny. After seizing control of the boat, the criminals take over and shoot the doctor. Impersonating his uncle, Bob broadcasts a warning to the killers, telling them that they will all die without their next injection. When one of them drops dead, the others rush to the doctor's office, where Bob and the orderlies take them prisoner. As they sail home, the doctor's experiments are vindicated when Mary awakens from her treatment and repents her crimes.