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In New York City, customs agent Cliff Holden learns that his colleague, Steve Regan, who had been investigating a ring of jewel thieves, has been murdered. Cliff also learns that around the same time, Lydia Brundage purchased an insurance policy for a $500,000 diamond necklace, then reported it stolen in France two weeks later. Cliff's superior, Inspector Brandon, tells him that one of the suspected thieves of the necklace, Matthew Royal, will be flying from Paris to New York, then gives him an airline ticket for a seat next to Royal's. Cliff takes his phony identification establishing him as "William Hara" and immediately leaves for France. While waiting to board the plane in Paris, Cliff meets a Dutch woman, Kathe Van Boren, who is flying to New York to marry her fiancé Joe. In New York, meanwhile, Joe is shot to death in his apartment by gangsters Birdie Alton and Owney Shore, who have broken in to steal Kathe's immigration papers. Back in Paris, Cliff and Kathe board the plane, then realize that they are sitting across the aisle from one another. When Royal takes the seat next to Cliff's and notices Kathe leaning across to speak with him, he offers to trade seats with her. Sometime after takeoff, Kathe goes back to use the rest room, and is followed by "Doc" Spencer, a disgraced surgeon who is now working for the gang. Doc knocks her out, injects her with a sedative and returns to his seat. When an unconscious Kathe is discovered by the flight attendant, Doc steps forward, "diagnoses" shock, orders an ambulance to be waiting for her when they land, and then puts Lydia's necklace into a bandage and wraps it around her head. After the plane makes an emergency landing, Kathe is rushed into an ambulance, and because they have her immigration papers, Birdie and Owney are permitted inside with Kathe. On the way to the hospital, Kathe begins to wake up, so Owney knocks her out with the butt of his pistol. Birdie tries to grab the necklace, and after the ambulance crashes, Kathe stumbles away, finds the necklace and arrives at Joe's place in a taxi. There, Cliff tells her what happened to Joe, and Kathe explains that she was attacked on the plane, never seeing her assailant. Cliff then tells Kathe to go to the home of his friend, Minnie Hoffman, who will help her hide from the immigration officials who will likely now pursue her. Later, Owney and Birdie break into Joe's place, and after Owney overhears Joe speaking about Doc, Owney escapes and finds Doc at his hotel. Posing as a fellow guest, Cliff goes to Doc's room, where he admits that Royal murdered Steve. Just then, Owney arrives and knocks Cliff unconscious. When he wakes up, Cliff is at Lydia's home, and she introduces herself. She says that while she was in Paris, Royal offered to sell her the stolen necklace for $250,000, and she agreed, saying that she could pay him in New York. After Lydia then offers Cliff $100,000 for the necklace and is rejected, she leaves. Later, Cliff goes back to Lydia's place and while eavesdropping through a window, sees Owney demanding all the money at gunpoint, as Royal stands by. When Owney reaches for the money, Royal suddenly tosses a knife into his back, killing him. Cliff jumps through the window, subdues Royal and calls the police. Later, Kathe arrives with the officers, and Cliff arranges for his new sweetheart, Kathe, to be placed in Minnie's custody.