The Mob (Robert Parrish, 1951) PAL DVD5

During a rainy night, Johnny Damico buys an expensive ring to propose his girlfriend Mary Kiernan to marry him. When he leaves the pawnshop, he hears a gunshot and sees two men, one standing and the other lying on the street. The man gives his badge to Damico e identifies himself as police detective. Damico gives attention to the man on the street but he is dead. When Damico calls his chief Lt. Banks, he is ordered to go immediately to the precinct, where he learns that the victim was the witness of a case against the hitman Blackie Clegg, who killed the detective that was investigating him first and showed his badge to Damico. Now Damico is forced to work undercover to find the whereabouts of Blackie Clegg. He has to report directly to Lt. Banks since there is corruption in the police force. He assumes the identity of the tough longshoreman Tim Flynn, from New Orleans, and begins his investigation befriending the stevedore Tom Clancy that lives in his low-budget hotel, and the bartender Smoothie in the beginning of his assignment.