When cafe owner Hartman refuses to pay graft to Little Joe Macey's gang of racketeers, his business is destroyed and he is beaten up. Bull Regan, a policeman, is passing by the cafe as the gangsters are leaving and arrests them. In court, however, Hartman succumbs to threats and refuses to identify the criminals. The case against Macey's gang is dismissed, and Hartman is sent to jail for perjury. In response, Inspector Robert L. Lane is appointed special prosecutor. He calls for the city's businessmen to testify against the racketeers, but most are too afraid for themselves and their families to do so. When Regan is assigned to Lane's office, Lane cautions him against bullying the witnesses, adding that a policeman's job is to protect the public. Lane advises him to find witnesses, gain their confidence and then persuade them to tell their stories in court. Regan is sure that his tactics are the right ones, however. He questions Frank Wagner, the owner of a neighboring cafe, and his aggressive attitude convinces Wagner that Regan is part of Macey's gang. Wagner runs to Lane's office, where Lane persuades him to testify. Although Wagner identifies the members of the gang from photos in Lane's office, on the stand, he backs down because of threats Macey made to his wife Gladys. This time, however, Lane is prepared and receives permission to show a film, taken secretly, of Wagner identifying the photos, thereby gaining a conviction. The man behind the gangsters is still at large, so without Lane's permission, Regan visits Macey in prison and offers him a deal if he reveals the name of his boss. Their conversation is overheard and Macey is killed. Lane is furious but decides to give Regan one more chance. They get a break when Mary Norton comes forward with her belief that her boss, Ward Sturgis, is the man behind the gangs. When they investigate her story of a gun battle on Sturgis' yacht, however, they find no signs of a struggle and the books that Mary claims reveal Sturgis' participation in the rackets, have been replaced with fakes. Then a body is found in the bay, and although his features are unrecognizable, the corpse is identified as Sturgis. Mary is then accused of his murder, and Lane sends Regan to arrest her. Regan finds her hiding in a closet, but decides not to arrest her. When he returns to the room to hear her story, she is gone. Later, her purse and some of her clothes are found on a ferry, indicating that she may have killed herself. Regan does not believe that Mary is dead and announces her arrest, hoping to lure her out of hiding. His ruse works and Mary is captured. She tells Regan that Sturgis is not dead, the body was really that of Jennings, her co-worker. As they are leaving the station, they see Sturgis and follow him. Reagan finds him at a hotel, packing to leave. He claims to be a respectable citizen with a different name, but after Regan brings him into the office, Lane tricks him into confessing and the way is cleared for a romance between Regan and Mary.