Michael Lanyard, a reformed jewel thief known as The Lone Wolf, and his butler, Jamison, are on their way to the Havana airport for their trip back to Miami, where they intend to attend a stamp collectors' convention. Moments before leaving Cuba, however, Lanyard and Jamison witness Patricia Lawrence being forced into a car by two men. They come to her aid, but in the melee, the men steal Lanyard's valuable stamp collection and disappear. Patricia later tells Lanyard that the men were after the $100,000 ransom money she was carrying for the release of kidnapped millionaire Cyrus Colby. She also admits that she was planning to use the money to buy the testimony of an old Portuguese man who was going to identify the real kidnappers and thus exonerate her fiancé Scotty, who has been accused of the crime. After convincing Patricia to allow him to handle the case, and eluding Inspector Crane, who believes he is involved in the kidnapping, Lanyard, Jamison and Patricia visit the old Portuguese man. Just as he is about to utter the kidnappers' names, however, a shot rings out and he is killed. In the ensuing scuffle, Lanyard is captured by the kidnappers, but Jamison and Patricia escape unharmed. Taken to Big Joe Brady's casino, Lanyard offers to cut Joe in on the ransom if he allows Jamison to come and pick up Lanyard's stamp collection. When Jamison arrives, Lanyard instructs him to enter the stamps in the Miami convention, but to pay special attention to the Cuban stamps. Unable to find the convention, Jamison examines the Cuban stamps more closely and finds a secret message inscribed by Lanyard. On the stamp, Jamison reads that Colby is being held prisoner on Sandy Key, and that he is to decoy the police there. Jamison follows Lanyard's instructions and leads the police on a chase to the key, where they soon find and arrest Big Joe and his henchmen. The police find the ransom on Big Joe and immediately release Scotty.