In Mexico City, U.S. Treasury Department investigator Patrick Nevil is assigned to follow newspaper columnist Agnes Stuart and report her activities to Secret Service agent Graeme. Patrick, a playboy, fears that his assignment will interfere with his romantic pursuit of Audrey Latham, a young woman from Seattle, until he meets and falls instantly in love with Agnes. Refusing to give up on Patrick, Audrey, accompanied by her wealthy friend, George Richards, joins him and Agnes for an evening of fun at the El Vaquero nightclub. There, Agnes tells Patrick that the club owner, Doctor Lilley, is an American racketeer who ordered the killing of her father. Patrick discovers the true purpose of Agnes' visit to Mexico City when she tells him that she plans to expose Lilley by setting a trap for him. George, who is a friend of Lilley, offers Patrick money to leave town and cease his investigation, but Patrick refuses the bribe. Patrick soon realizes that he has stumbled onto a dangerous racketeering operation when Manuel Perez, Agnes' driver, is found murdered. Believing that Agnes is in a great deal of danger, Patrick tries to dissuade her from executing her plan, but she responds by accusing him of being an operative for Lilley. During their conversation, Agnes tells Patrick that she suspects that George is really a counterfeiter named Freddie Nigh. Later, Audrey tells Agnes that she and Patrick are engaged, after which Agnes, devastated by the news, goes to Lilley's. Patrick follows her there, and he and Agnes discover Lilley's plan to flood Europe with counterfeit bills. Patrick and Agnes are then caught searching Lilley's house, and are about to be killed when Audrey arrives, unaware of the danger they all face. Lilley sentences Audrey to die with the others, and assigns a guard to carry out the executions while he plans to flee the country. Patrick manages to overpower the guard and free Audrey and Agnes, but the three are quickly recaptured by George. While attempting to transport his three prisoners back to the house, George is arrested by Graeme and the Mexican police. With George, Lilley and the other racketeers in police custody, Audrey tells Agnes that she was lying about her engagement to Patrick, leaving Agnes free to resume her romance with the secret agent.