The Duke of Sairmuse weds Blanche Courtleigh. Shortly alter she is blackmailed for money by a band of ruffians, who threaten to inform the Duke that her brother is a hardened convict. Wild with anxiety she decides to meet the blackmailers and strive to effect a compromise. Her husband, suspicious of her actions, follows her and has one of the gang try to steal her earrings. He breaks in and in a struggle kills them. He is disguised in old clothes to save his wife's honor and holds off the police until she escapes. Monsieur Lecoq, the famous detective, captures the Duke, but to protect his wife he keeps silent. The famous detective tries various devices to entrap the nobleman and learn his real identity, but it is useless. At last they permit him to escape and follow him to his home. While the Duke's faithful servant, Ott, is parleying with the sleuth, the Duke changes his clothes and confronts him in his natural guise. Baffled, the hound of the law departs.