In San Francisco, California, Rosa Moreland of the Secret Service is unable to obtain evidence against suspected opium smuggler George Sala. She then advises Detective Flint of her plan to develop a relationship with John King, an impoverished, disabled ex-soldier who Rosa met in George's office. After claiming to have lost her home in a fire, Rosa is invited to stay in John's modest flat. He receives money from a mysterious source, enabling them to afford a more expensive apartment, and they soon fall in love. Although Rosa secretly witnesses John receiving a package from George, she tells the skeptical Flint that her lover is above suspicion. John agrees to end his association with George if Rosa will marry him, and, realizing that a wife cannot legally testify against her husband, she agrees. Flint anticipates the wedding and sends a squad of police officers to interrupt. George, who has learned Rosa's true identity, is another unwelcomed intruder. She discovers that John is also a Secret Service agent, and that he has obtained enough evidence to convict George.
—AFI