In Cairo, The Partner, a member of a gang of international jewel thieves led by Boss Duke Bates, ships a mummy to Professor Horatio Bitts in Philadelphia under the name Simon Templar. By coincidence, Inspector Henry Fernack of New York is visiting John Bohlen of the Philadelphia police department when Bohlen is called to Bitts' house to investigate a murder. Private detective Simon Templar, known as "The Saint," had been visiting the professor, an old friend, earlier that evening, and when Fernack finds a card belonging to the Saint on the murdered man's body, Bohlen concludes that Templar is the murderer. Later that night, Templar visits Fernack in his hotel room, where the inspector confides that he believes him to be innocent. After leaving Fernack, Templar returns to the scene of the murder, and meets the professor's daughter Anne and inspects the mummy. Unknown to Templar or the police, Boss Duke Bates is Templar's exact double and has been impersonating Templar. After Boss kills a jewelry fence, Templar traces the fence's murderer to the Boss's hideout at the 4 Bells Cafe and discovers that he has a double. Meanwhile, Boss learns that there is another package of gems concealed in the mummy's body and returns to Bitts's house, where he kills the professor. Templar, still evading the police, sends Anne and Fernack messages directing them to the 4 Bells Cafe. Boss intercepts Anne, however, and orders her elimination, but Templar saves her life at the last moment. Templar then returns to Boss's headquarters, where he is captured and thrown, tied and gagged, onto a boat. As Boss plans Templar's demise, Fernack and the police arrive. Believing that Boss is the Saint, the police arrest him, and after they leave, Templar escapes from the boat. To regain the jewels and prove his innocence, Templar, disguised as a woman, visits Boss in his cell and allows Boss to overpower him, don his disguise and escape. When Boss is shot down by a police officer, the Saint's "double trouble" comes to an end, and Fernack allows Templar to escape after he returns the jewels.