Retired gem merchant Peter J. Martin hires detective Dan McRae to guard a large diamond which he plans to present to his daughter Ruth on her twenty-first birthday. Notorious jewel thief Gentleman Joe overpowers McRae, however, and takes his place at Ruth's party. Meanwhile, Martin's nephew, Robert Kendall, who plans to steal the diamond to repay heavy debts, enlists Ruth's brother Harold as an unwitting accomplice. The plans go awry, however, resulting in Martin's murder by Kendall, secretly witnessed by Joe. Harold is found next to the body and later confesses to patricide to stop an intense police interrogation. When Joe hears that Harold has been charged with murder as well as the theft of the diamond, he determines to help him. Joe confronts Kendall, offers him a gun and gives him the choice of arrest or suicide. After writing a full confession, Kendall kills himself, thereby exonerating Harold. In his apartment, Joe entertains McRae and presents him with a jar of tobacco in which the diamond is hidden.