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While walking home late one rainy night, off-duty police detective Johnny Damico hears the gunfire of a shootout. A man at the scene identifies himself as a fellow officer and hastens to summon police backup as Johnny examines the victim. When the officer fails to return, Johnny grows suspicious and telephones his commanding officer, Lt. Banks. After Johnny relates the events, Banks summons him to headquarters, where he informs Johnny that the badge shown to him by the man at the scene was taken from an officer murdered a few hours earlier. The victim is identified as the principle witness for a grand jury investigation into the waterfront rackets. Although Johnny cannot provide a clear description of the murderer and is chastised for letting him escape, Banks assigns him to go undercover on the waterfront to find the top rackets man, who is known only as Blackie and is the suspected killer. Publicly, the police declare Johnny suspended and run a phony picture of him in the newspaper. Johnny is to pose as Tim Flynn, a small-time crook from New Orleans and, before leaving for New Orleans, he visits his girl friend, Mary Kiernan, and proposes, telling her only that he will be out of town for some weeks. Soon after, Johnny returns from New Orleans by freighter and, as Flynn, checks into a cheap waterfront hotel, The Royal, where he meets Tom Clancy, a longshoreman. The next day, Johnny secures a work permit and Tom advises him that kickback payments are required in order to obtain work. Having overheard several men mention the name Joe Castro, Johnny repeats the name to the dock manager when he checks in for work and is assigned an easy job. The worker he supplants, Culio, reacts angrily at being replaced and threatens Johnny. That evening, a man named Gunner forces Johnny into a car and takes him to see the dock manager and the apparent racket boss, Castro, who questions him at length. The next day on the dock, Johnny is arrested for the murder of Culio, who was killed with a gun like his own. After being harshly grilled at headquarters, Johnny suspects Sgt. Bennion is working in collusion with Castro. When Banks discovers that "Flynn" has been arrested, he interrupts the questioning and soon clears Johnny with a ballistics report. Johnny divulges everything he has discovered about Castro to Banks, and states his belief that he is not Blackie. Banks then advises Johnny to investigate Tom. Back at The Royal, Smoothie, the bartender, repeats a rumor circulating the docks that Johnny was set up for Culio's murder and tells him that Gunner was involved. When Johnny is unable to force information from Gunner, he has Banks arrest him for Culio's murder without any publicity. The next day Tom invites Johnny for an evening out with two friends, Doris and Peggy. Unknown to Johnny, Peggy is Mrs. Clancy and her husband has asked her to get Johnny drunk and find out more about him. After Johnny passes out at Tom's, he awakens the next morning in his room at The Royal with an armed Castro standing over him, demanding to know Gunner's whereabouts. Tom bursts in and knocks Castro unconscious and calls Johnny by his real name. As they bind Castro up, Tom reveals he is a government investigator also working on the rackets scam and has just identified Johnny through fingerprints. While Tom places Castro under arrest, Smoothie tells Johnny that someone has asked to hire him to kill a cop. Johnny reports to Banks that he has made contact with Blackie, and Banks arranges to place a transmitter in Smoothie's car before he drives to the meeting. Inadvertently, however, while tracking Johnny, the transmitter's signal is obstructed. At the meeting place, Smoothie reveals that he is Blackie and informs "Flynn" that he has had Mary kidnapped and tortured to discover Johnny Damico's identity and whereabouts. When Johnny tries to rescue the injured Mary, Smoothie escapes, but not before being wounded by Johnny. A few days later in the hospital, Johnny's visit to Mary is interrupted by Smoothie, who is convalescing down the hall. Realizing Johnny's identity at last, Smoothie is about to kill him, but is shot from the neighboring building by a police sniper. His reputation on the force restored, Johnny and Mary celebrate with Tom and Peggy.