Dr. Dudley Duprez is a well-known Louisiana physician. His beautiful but wayward niece, Rose Duprez, is abducted by Paul Crenshaw, a friend of the doctor, and to prevent her shame from becoming known, Rose kills herself. Dr. Duprez learns her secret and determines to make Crenshaw expiate his crime. While traveling on a Mississippi River steamer, the doctor wins Mercedes, a beautiful slave, at cards. He takes her home and, passing her off as a distant relative, arranges it so that Crenshaw falls in love with the girl. A wedding is arranged, and immediately after the ceremony Dr. Duprez announces to the assembled guests that Mercedes is a slave and that he considers he has punished Crenshaw sufficiently by making him the husband of a "nigger." A yellow fever epidemic breaks out shortly after, and Crenshaw is shot when attempting to evade the quarantine. Dr. Duprez is told by a dying overseer from the plantation where Mercedes was born that the girl is of Spanish and American ancestry, without a drop of negro blood in her veins, and was made a slave through a conspiracy. The doctor returns home, confesses his wrong to Mercedes and is forgiven by her. In the end they are married.