A man dodges jewel thieves while carrying a fortune in diamonds on the Orient Express.

A series of jewel robberies have the Paris police and insurance companies baffled. In desperation, Hoyle's Insurance Company decides to use the famous Karenina diamonds as bait to lure the thief into the open. During the auction of the diamonds, Pierre Londais outbids Odette Mauclair, an attractive young woman. Just after the sale is completed, Colonel Jackson rushes into the room and tries to buy the jewels from Pierre, but he will not sell. Back at his hotel, Pierre discovers that Odette has the suite next door and they dine together. That night, alerted by a noise, she enters Pierre's room and is met by a gloved man. Although the jewels are missing and Pierre's valet, François, is injured, Pierre refuses to call either a doctor or the police. Odette is puzzled by his attitude, and he wonders in turn why she was in his rooms that night. The next morning, Pierre reveals that the diamonds were in the false bottom of the case holding the jade he loaned her and so were not stolen. When Pierre takes an Istanbul-bound train to deliver the diamonds to a prospective buyer, Odette follows carrying a copy of the jewels. Colonel Jackson also follows Pierre to the train, and Pierre finally agrees to sell him the diamonds. When Odette expresses her surprise, he shows her a telegram in which the earlier offer for the jewels is canceled. Pierre, then confesses that he has fallen in love with Odette and asks her to return to Paris with him, When she refuses, he remains with her. Meanwhile, Colonel Jackson insures the diamonds for a great deal of money. That evening, Odette sneaks into his compartment and exchanges the fake diamonds for the real ones. Baron Von Kampf, another thief, sees her leaving Jackson's compartment and demands the jewels, but she manages to escape from him. The next morning, Jackson announces the theft of his jewels. The baron accuses Odette, who agrees to be searched by investigator Count Trentini. Out of sight of the others, she hands Trentini the jewels, revealing that she is working secretly for the police. When the rest of the train is searched without turning up the fake diamonds, Odette is convinced that Jackson robbed himself for the insurance. A look at a telegram he sent seems to confirm this theory, but then he is found dead. Pierre finds a bag containing the jewels in the seat of his compartment, but Odette refuses to believe Pierre murdered Jackson for the jewels. When she sees a man jump from the train, she pulls the emergency cord and stops the train abruptly. The man is captured and admits that he killed Jackson because he thought he was cheating him after he discovered the jewels had been replaced by fakes. He implicates another man but dies before he can name him. Through further investigation, Odette learns that Pierre was the front man, buying the jewels that were then insured by different people, so that his name was never involved. Before Pierre can be arrested, the baron steals the jewels, informing everyone that he has arranged a fatal train accident during which he will escape. Pierre shoots him, allowing the police to stop the accident. Pierre is sentenced to jail, but his sentence is suspended because of his actions on the train. Odette and Pierre leave the train together.