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Simon Templar, the debonair sleuth known as "The Saint," is hounded by the press for a story as he sets sail for a vacation in Switzerland with his friend, Monty Hayward. Determined to get a story, reporter Mary Langdon trails Simon to the Hotel Regina and corners him in the lobby. There Simon excuses himself to greet Valerie, an old friend, but knows something is amiss when she denies knowing him and furtively sneaks away to meet a man she calls Gregory. Simon watches as Gregory speeds away in his car and is followed by Rudolph Hauser, Simon's old adversary. Simon then proposes that Mary and Monty accompany him on a stroll in the woods, where they witness a man being assaulted. Simon comes to the rescue and takes the injured man to his hotel suite.
After finding a locked box in the man's possession, Simon goes into the next room to show it to his companions, and when he returns to his room, he discovers that the man has been stabbed to death. When Hauser and his accomplice Marko appear and reclaim the box at gunpoint, Simon jumps onto the rear bumper of their car and follows them to their headquarters, where he watches as Hauser interrogates Gregory about the box. Simon then steps in to claim the box, calling Mary with instructions to meet him at the St. Gallen crossroad. As Simon speaks into the phone, Hauser grabs the box and disappears through a secret panel. Hauser and Marko drive to the crossroad, where they plan to eliminate Simon, but are surprised when Simon pops out of their trunk and demands the box. Just then, Monty, disguised as a police officer, drives up and, after returning the box to Hauser, pretends to arrest Simon. As they drive away, Simon informs Monty that he emptied the contents of the box, and thus to Monty's chagrin, the troublesome object is still their traveling companion.
At the hotel, Simon examines the music box that he removed from Hauser's box and discovers that its playing cylinder punches a peculiar code on a piece of blank paper. When Valerie calls and demands the box, Simon decides to mail it across the border and books passage on the train that carries the mail. Also on board the train are Mary, Monty, Hauser, Marko, Gregory and Valerie. Rudolph and Marko begin to rifle the mail car but are interrupted by Gregory, who is killed in the ensuing shootout. Nevertheless, Rudolph finds the box and when the train stops at the next station, the police, who have been alerted by Hauser, board the train and arrest Simon for the murder of the man found in his hotel room. When Hauser opens the box, he finds a medicine kit and, realizing that Simon has tricked him, abducts Monty and calls Simon in jail to demand the box in exchange for Monty's freedom.
Meanwhile, Mary goes to visit Simon in jail and is arrested as his accomplice. Feigning a fire in their cell, the two escape and drive to Hauser's castle. Simon realizes that Hauser is unaware that the box is stowed in Monty's luggage, and offers to tell him the location of the box in return for clearing Simon's name and releasing Monty. Hauser agrees to Simon's terms, and after Mary speeds off to Paris with Monty, Simon informs Hauser that the box is on its way to Paris and threatens to charge Hauser with murder and mail theft unless he is allowed to go free. As Simon returns to London, he is greeted at dockside by the story-hungry press and Inspector Teal, who accompanies Simon, Mary and Monty to headquarters, where Simon turns the box over to Charles Leighton of the war office. As Leighton explains that the notches on the box's music cylinder replicate a plan for a secret sound detection device, Valerie, who was also working for the British government, enters the room and greets Simon.