When King Carney, a grand jury witness, is gunned down before he can testify, gangster Louie Butler is suspected. After a manhunt, Butler is arrested, but is later found hanging in his cell. Police Chief Sprague is skeptical, believing that Butler was not the kind of man to hang himself. Together with Joe Geary, the only policeman on the force whom Sprague trusts, he devises a plan to root out corruption in the city. As part of the plot, Joe is publicly accused of consorting with criminals and suspended from the force. He is taken in by Crips Vossen, who collects from businesses running crooked gambling games. Joe's job is to learn the identity of "Lockland," the head of the crime syndicate. When Joe proves himself to Vossen, he is taken to meet Lockland, who is revealed to be police captain Reddick. With his henchman, Lieutenant-Detective Pagle, Reddick was responsible for the deaths of Carney and Butler. Joe forcibly takes Benny McKaye, one of the men working for Reddick, to testify for Sprague, but they are followed by Pagle, who then kills Sprague and wounds Joe. Benny escapes and Pagle puts the gun that killed Sprague in Joe's hand. Without any witnesses, no one believes Joe's story that he was working undercover, and he is sentenced to life in prison. Joe's fiancée, Alice Devlin, is convinced that he is innocent, and together with their old friend Katie, a victim of the syndicate, searches for Benny, who is Joe's only hope. In prison, Joe is victimized by the guards and the other criminals. Some time later, Durkin is sent to prison for killing Vossen. During a fight with brutal guard Monson, Durkin is badly wounded, but before he dies, he tells Joe that Katie has found Benny. Desperate to clear himself, Joe escapes from prison and heads for Katie's, but before he arrives, Benny dies without talking. Captain Allen, an honest policeman, follows Alice to Katie's and warns Joe that Reddick and Pagle are looking for him. Learning that the reform-minded governor has vowed to clean up the city, Joe borrows Allen's badge, coat and hat and drives Benny's body to the governor's hotel. There he asks the governor to phone Reddick to tell him that Benny is on his way. Joe and the governor witness Pagle and Reddick shoot at the car with Benny's body in it. After the governor tells Reddick that Benny telephoned him before he was killed and cleared Joe's name, Reddick agrees to testify. Pagle attempts to shoot his way out, but Allen kills him. Joe's name is cleared and he is assigned to head the graft investigation.