When a priceless collection of jewels assembled by King Frederick of Brandenburg, seized by the U.S. occupying government in Germany, is stolen, suspicion falls on Marcel Grandet, who was an informant for both sides during World War II. College music teacher Johnny Vickers, who was a secret agent during the war and knows Grandet, is assigned to recover the jewels and heads for Lisbon where Grandet operates a nightclub. Johnny tells Grandet and his wife Claudette that since he last saw them, he has been in Buenos Aires. Grandet gives Johnny back his wartime job as the club's piano player. Club cigarette girl Carmelita Mendoza pretends that she is not pleased to see Johnny, who is an old boyfriend, but she, too, is an agent. Grandet checks on Johnny's story, and as Johnny has arranged for a telegram from Argentina to back up his claim, Grandet is convinced. Former wartime colleagues Roger Hamilton and Miguel Solomon, a jeweler, assist Johnny's investigations, while Pablo, Carmelita's young brother, spies on Grandet and his associates and tells Johnny that he followed Grandet and saw him receive jewels from a man named Koslic. Johnny tricks Grandet into believing that he has sufficient funds to buy the Brandenburg jewels. Claudette, meanwhile is angry with her husband when she learns that he intends to break up a necklace that once belonged to Queen Marie Antoinette, and sell the stones individually. Later, when Johnny goes looking for the jewels in the nightclub's wine cellar, he is surprised by bartender Pierre Dijon, who becomes an ally. Eventually Grandet invites Johnny to make him an offer for the collection, but their negotiation is interrupted when Grandet shoots and kills Pierre, whom he has discovered has been double-crossing him. To test Johnny, Grandet takes him to the cellar, where he has Hamilton tied up, hands Johnny a gun and orders him to shoot Hamilton as a gesture of good faith. When Johnny pulls the trigger, he discovers that the gun is not loaded. Grandet then breaks open a wine vat expecting to recover the jewels from their hiding place but they are not there. Grandet immediately suspects that Johnny has found the hiding place but then realizes that Claudette followed him when he hid the jewels and shoots her. Johnny jumps on Grandet and, while they fight, Carmelita sends Pablo to fetch the police. As one of Grandet's henchmen is about to hit Johnny with an axe, Carmelita shoots him, after which Johnny overpowers Grandet. With her dying breath, Claudette admits to taking the jewels, as she could not bear to see them broken up, and tells Johnny where he can find them. Later, when Hamilton asks Johnny if he knew that the gun he fired at him was not loaded, Johnny replies that he did not but assumed that Grandet would not be fool enough to hand him a loaded gun and adds that even if he had, Hamilton would have only lost the tip of his left ear.