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In New York City, Meek accountant Sam Carter has dreamed for years of possessing a famous diamond known as the Tyrahna Blue. After years of planning, Carter decides the time is right for him to steal the diamond from the jewelry concern for which he works. During a brazen robbery, Carter kills a guard, then makes off with the diamond. The insurer of the gem, the Midwest Insurance Company, knows that every international jewel thief will be on the trail of the amateur, and so hires private detective Milo March to find him first. The only clues that March has to go on are a nondescript photograph of Carter and the knowledge that he speaks Spanish. When March visits Carter's former lodgings, he finds them occupied by a glamorous blonde, Trudie Hall. Their discussion is interrupted by the explosion of a bomb that a would-be assassin placed in March's car. When March returns from surveying the wreckage or his car, Trudie has vanished. Following a chance remark made by Carter's landlady, and the signature on a painting found in Carter's rooms, March locates the convict whom Carter bribed for the name of a passport forger in Lisbon. Meanwhile, upon arriving in Lisbon, Carter realizes that he has been followed by Martin Lomer and Rizzio, two cutthroats who plan to steal the diamond for themselves. When the forger, Tristao, demands the jewel in return for a new passport, Carter shoots him and hastily leaves Lisbon. March arrives in the Portuguese capital just in time for Tristao's funeral. Proceeding to Carter's last hotel room, March finds Trudie there holding a pistol in her hand pointed at March. After disarming Trudie, March forces her to reveal why she is following Carter. Trudie claims to be the rightful owner of the diamond, which she says, was stolen from her father by Nazis during the occupation of Austria. A tip from a friend of Tristao's sends March to Madrid, where he hires Ernesto, a taxi driver. While driving past a statue of Don Quixote, March recalls seeing the same design on the cover of a book he found in Carter's lodgings, and consequently decides to take a room in a nearby hotel. In the hotel bar, March hears a stranger being addressed as Señor Sanson Carrasco. Remembering that Carrasco is a name associated with the history of Don Quixote, March correctly suspects that Carter may be masquerading as Carrasco and strikes up a friendship with him. March's plans are upset by the arrival of Trudie, who uses her charms to attract Carter. March warns Trudie that her association with the thief may prove to be dangerous, a prophesy that comes true when the detective finds Trudie brutally stripped and beaten by Rizzio and Lomer, with whom she was secretly in league. Feeling a mixture of love and pity for Trudie, March promises to help her obtain the stone legally once it is retrieved. The next day, March, Trudie and Carter are trapped on a side street by Rizzio and Lomer. March diverts the thugs while Trudie escapes with Carter, and later learns that Trudie has run off to Paris with Carter. In Paris, Carter nearly strangles Trudie when she demands the diamond. Later, after March arrives, Carter finds him embracing Trudie and tries to kill them both. After his murder attempt fails, Carter flees and boards the boat train to London. Rizzio, Lomer, Ernesto, March and Trudie follow. Once on board, Rizzio holds March, Trudie and Ernesto at gunpoint while Lomer tracks down Carter. After March overpowers Rizzio and pushes him off the train, Carter is apprehended by Lomer. Ernesto and March try to save Carter, who is being held at knife point by Lomer. In the ensuing scuffle, Carter breaks free, rushes to the window and holds the diamond out, threatening to drop it. Carter drops the diamond inside the train compartment after Lomer fires his gun at him, fatally wounding the amateur thief. March then trips Lomer, sending him plunging through the window. After retrieving the diamond, Trudie finally admits that the jewel never belonged to her family, but nevertheless appeals to March to escape with her and the diamond. He refuses, and upon arriving in London, discovers that Trudie has only given him the diamond's container, keeping the gem for herself. Trudie soon appears and presents him with the gem, having realized that March means more to her than the diamond.