Simon Templar, the debonair sleuth known as "The Saint," becomes involved in murder when his friend, New York City police inspector Henry Fernack, asks him to act as bodyguard for Peter Johnson, the brother of an old friend who is carrying three rare stamps valued at $200,000. Johnson has smuggled the stamps, which represent his brother's life savings, out of Europe to present to his niece, Elna Johnson, a tennis pro at a resort in Palm Springs, California. Simon agrees to accompany Fernack's friend to Palm Springs, but as Peter withdraws the stamps from his hotel room safe, a gunshot shot rings out and he falls to the ground, dead. Simon strikes Peter's assailant in the face with his "Saint" ring, but the man escapes, leaving the stamps behind. Simon then boards the train to Palm Springs, where he meets Margaret Forbes, a fellow passenger who is also bound for the Twin Palms Lodge in Palm Springs. At the lodge, Simon meets his old friend, pickpocket Clarence "Pearly" Gates, who is working as the house detective while on probation from prison. When Simon is assaulted in his bungalow by a thief in search of the stamps, he informs Pearly that he has been followed from New York and asks for his help. The next day, Pearly introduces Simon to Elna, but when he presents her with the locket in which the stamps have been hidden, she discovers that the locket is empty. Simon asks Elna for twenty-four hours to find the stamps, and after obtaining a list of the hotel guests who had checked in the previous day, he asks Pearly to pick their pockets, hoping to find the stamps. As they examine their lot, Simon finds the stamps in a pillbox containing knock-out pills. When Pearly is unable to remember from whose pocket he procured the pillbox, Simon asks police chief R. L. Graves to help set a trap for the murderer. That night, the pickpocket victims are summoned to a room to reclaim their losses, but before the owner of the pillbox can be identified, a police officer is murdered and the box disappears. Soon after, an armed man reclaims the box from Simon, but the sleuth had already removed the stamps from the box. The next morning, Simon presents the stamps to Elna, who locks them in the hotel safe. That afternoon, Elna, Simon and Margaret go horseback riding and Simon advises Elna that the stamps would be safer in her possession. When she returns to the lodge, Elna withdraws the stamps from the safe, and that night, Pearly and Simon watch her bungalow in hopes of trapping the murderer. When Elna returns to her bungalow, Margaret demands the stamps at gunpoint, but after Margaret runs outside, a shot rings out and she falls to the ground, dead. Simon then shows Elna a telegram that he had received from Fernack, informing him that Margaret was a foreign agent in search of the stamps. Margaret's death leads Simon to conclude that they are now dealing with an organized gang, and he sets another trap for the murderer by sending a note to Elna instructing her to meet him at the Joshua tree at midnight. The murderer intercepts the note, and that night, as Pearly and Elna wait for Simon, Mr. Evans, a guest at the hotel, invites Simon for a drink and drugs him with knock-out pills. Failing to find the stamps in Simon's possession, Evans orders one of his men to guard Simon while he and the rest of the gang drive to the Joshua tree. At the tree, as Evans threatens Elna, Simon, who had substituted aspirin for the knock-out pills, rides up and tricks Evans into confessing to the murders. At that moment, Chief Graves and his men appear from the shadows, and when Simon reveals the scar that the blow from his ring inscribed upon Evans' face, the case is closed.