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Los Angeles police detective Scott Harper and his partner, Murray Kearns, are assigned by Inspector Dan Melrose to investigate the robbery of uncut diamonds worth $250,000 from a wholesale jeweler. With the help of insurance investigator Tom Colton, Harper and Kearns establish a list of suspects and narrow it down to a gangster named Kurt Romack. After Colton suggests that they also check on the activities of a notorious fence, Martin Bender, Melrose sends Kearns to San Francisco to find him. When one of the jewelers dies from wounds inflicted during the robbery, the robbery charge becomes murder. Upon discovering that Romack's girl friend, Holly Taylor, is a hostess at a local nightclub, Harper poses as an out-of-town visitor and romances her. The next day, Harper learns that he has been passed up again for a promotion to lieutenant, despite a good recommendation from Melrose, who suspects that Harper's proclivity for violence and irregular methods work against him. Later, Holly tells Harper that she has discovered that her apartment is under police surveillance and asks to leave town with him. Harper then questions her about Romack and, when she is astonished that he knows about her gangster friend, admits that he is a police officer and that, although he was using her to get information, he has now fallen in love with her. Harper suggests that Holly can help him catch Romack, but she fears Romack and instead proposes that Harper could kill him "in the line of duty while resisting arrest." At first, Harper refuses but, disillusioned by his lack of promotion and the news that two of his fellow officers have been shot, decides to go along with Holly's plan. Holly tells Harper that she is to set up a meeting between Romack and Bender at a Los Angeles motel, where the diamonds will be exchanged for cash, and notes that the meeting would provide an opportunity for him to shoot Romack during an attempted arrest. When Harper says that he intends to return the diamonds to their owner, Holly convinces him that they should keep the stones. Meanwhile, in San Francisco, Kearns and local officers have located Bender and are following him. In preparation for their meeting, Holly rents a motel room and mails a key to Bender. Later, as Romack enters the motel room, Harper emerges from the bathroom and kills him, then takes the pouch of diamonds from the body. Kearns, however, has intercepted the key intended for Bender and returned to Los Angeles. There, he overhears Harper and Holly discussing their future, enters and demands his partner's surrender. After Kearns refuses Holly's offer of half of the diamonds, she distracts him by throwing an ashtray at him, allowing Harper to shoot and kill him with Romack's gun. Harper then hides the diamonds in the drain of the bathroom sink, after which Holly leaves. When Melrose arrives on the scene, Harper tells him that he shot Romack after the gangster killed Kearns. Although Melrose wonders how Kearns got there and what happened to the stones, he accepts Harper's account. Harper is then lauded as a hero for eliminating his partner's killer and receives a medal and promotion to lieutenant. Harper convinces Melrose that he should continue seeing Holly as she may know the whereabouts of the diamonds. When Harper returns to the motel, he discovers that a plumber is about to disconnect the clogged drain, and after slugging him, retrieves the diamonds and then suffocates the manager when she enters unexpectedly. Holly, meanwhile, unaware that she is being followed by other police officers, flies to San Francisco to set up a deal with Bender. When Harper arrives at Bender's house with the diamonds, he in intercepted by officers on a stakeout who assume he is still on the case. Using one of the officer's phones, Harper calls Bender and tells him about the police surveillance. When Holly leaves Bender's house, Harper pretends to arrest her and persuades the other officers to let him take her back to Los Angeles. Once alone, Harper explains that he has bought tickets for a flight to Mexico City. After Colton informs the police that the plumber has identified Harper from a newspaper photograph taken at the time of his promotion, officers trace Harper's rental car to the airport and learn of his flight to Mexico. Harper and Holly are about to clear Mexican customs when the police receive a phone call to detain them. After a brief chase and exchange of gunshots, Harper is killed and Holly is arrested.