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F.B.I. agent Steve Emerson's exploits in capturing criminals lead to so much publicity and photographs in the newspapers that his boss finally tells him that he can no longer operate effectively. Taking the loss of his job philosophically, Steve decides to become a private detective, but his career almost ends there too when his first client is tied up and robbed just before their appointment. As it turns out, the Count de Grissac, his niece Lorraine, and cousin George Bouchet were bound needlessly as the criminal stole a paste imitation of the famous de Grissac emerald necklace. Steve is intrigued, however, when he finds a card with the name "Arsene Lupin" written on it and discovers that a bullet found in the wall comes from the type of gun used by the notorious and presumed dead French jewel thief Arsene Lupin. The de Grissacs return to France, accompanied by Bouchet and Steve, and are met by Rene Farrand, Lorraine's fiancé. Steve, who is attracted to Lorraine, is jealous of Rene, and also suspects him, but needs to find more proof to corroborate his hunch. Rene, actually the retired Lupin, did not steal the fake necklace, but must now try to find the real thief with the aid of two old pals, Joe Doyle and Alf, who come to see him, mistakenly thinking that he is no longer "going straight." After other attempts to steal the real necklace and after Steve and Rene play a cat-and-mouse game with the Prefect of Police, Bouchet turns out to be the real thief. Overwhelmed by gambling debts, Bouchet used Lupin's techniques to steal the jewels, then attempted to fence the necklace with Papa Monelle but killed him after Monelle discovered that Bouchet gave him the fake. Rene traps Bouchet by covering the de Grissac safe with lipstick, thus leaving the identifying red stains on Bouchet's gloves and neck when he tries again to steal the real necklace. Certain that Rene is Lupin, Steve now asks for his "autograph," but tears it up after Rene signs Lupin's name, realizing that Rene is an honest man and the thief Lupin will remain dead. Rene then tells Lorraine that Steve is the only man who has an open invitation to visit them after they are married.