After a New York undercover detective is killed while conducting an investigation in the Portuguese protectorate of Macao, the police commission in Hong Kong notifies the New York police about the murder. Later, on a Macao-bound boat, American passenger Nick Cochran rescues pretty Julie Benton from a masher, but instead of expressing gratitude, she brushes him off and picks his pocket. Upon docking in Macao, Julie informs customs officials that she is an out-of-work singer, while fellow American Lawrence C. Trumble describes himself as a casino-loving salesman. Nick then admits to Lt. Sebastian of the Macao police that his passport and money were stolen and that his only identification is his Signal Corps discharge papers. Sebastian tells Nick not to worry, but later shows Vincent Halloran, the American owner of the Quick Reward casino, a photograph of Nick taken at customs. The crooked Sebastian and Halloran, a racketeer who is living in Macao to escape prosecution in the U.S., conclude that Nick is another New York undercover detective and determine to stop him. Halloran also sees a photo of Julie and is deeply smitten. Nick, meanwhile, deduces that Julie is the pickpocket and goes to her hotel room to confront her. Julie denies Nick's charge, but when Sebastian arrives, looking to deport Nick for vagrancy, she slips him some of his cash. Sebastian then suggests to Julie that she seek work at the Quick Reward. After Julie secures a singing job with Halloran, the unsuspecting Nick also asks the racketeer for work. Nick admits that he has been drifting since getting into a scrape in New York five years earlier, and Halloran refuses to hire him. Eager to be rid of Nick, whose attraction for Julie he senses, Halloran tries to help him win at craps, but cannot bribe him to leave Macao. Instead, Nick invites Julie for a romantic sampan ride and tells her that recently he was offered a job running a plantation but, fearing loneliness, turned it down. When Julie, who has been hardened by a series of bad love affairs, expresses interest in the plantation, Nick declares that he will accept the position but that she cannot join him until he has gotten settled. Julie interprets Nick's suggested delay as a brush-off and demands to be taken back to the hotel. The next day, Trumble shows Nick a large diamond taken from a necklace and asks him to sell Halloran the necklace, which is in a hotel safe in Hong Kong. After Trumble promises him $10,000 if he succeeds, Nick approaches Halloran, who agrees to cross the three-mile limit that legally protects him and go to Hong Kong with Nick. Nick then persuades Julie of the sincerity of his emotions and reveals to Trumble, the actual undercover detective, whose real name is Lt. Brian, that he fled New York after he shot a man in a jealous rage. That night, at the docks, Nick is kidnapped by Halloran and learns that the necklace was stolen by Halloran before Trumble stole it from Halloran's fence. After discovering Nick's disappearance, Trumble sends a Morse code message to the Hong Kong police, who unwittingly wire Sebastian the next day. Upset about the communication and the necklace, Halloran decides to risk sailing to Hong Kong to meet with his fence. Julie, meanwhile, is led by a sympathetic old blind man to the house where Nick is being held. As soon as Julie sees Nick in the company of Halloran's girl friend Margie, she storms off, unaware that Halloran's thugs are threatening Nick with their guns. Margie, in turn, is jealous of Halloran's interest in Julie and allows Nick to escape, but he is pursued across the docks by Halloran's henchmen. Trumble catches up with Nick and gives him a gun just as one of the thugs throws a knife into his back. With his last breath, Trumble tells Nick about a police boat that is waiting for him and advises him to "fix things" with the New York police. Nick then finds Julie and, after convincing her of his fidelity, suggests that she accept Halloran's invitation to go to Hong Kong in order to distract him. With Margie's and Julie's help, Nick sneaks aboard Halloran's yacht and steers it toward the police boat, which is just outside the three-mile limit. As the boat crosses the limit, Nick fights with Halloran and knocks him overboard. After delivering the unconscious Halloran to the police, Nick swims back to Julie and proposes.