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When big city policemen are summoned to the site of a multiple murder, they discover the bodies of well-known gangsters.  Captain Jim Fitzpatrick, who heads the investigation, suspects mobster Sam Belmonte of the crime and arrests him at his club.  Though Belmonte is responsible, his lawyer Michaels gets him and his assistant, Pietro Cholo, released in a few minutes.

Jim, who takes risks and is dedicated to his work, is constantly at odds with the chief of police over his inability to control organized crime.  As a result, Jim is transferred to a quiet precinct out of town.  Jim's brother Ed, who is a vice officer, agrees to help get information about Belmonte and decides to question Daisy Stevens, Belmonte's stenographer.  After following Daisy to her apartment, Ed accepts her story that she has left Belmonte because of the murders and the two begin an affair.

Sometime later, as Jim's friends, Mac and Tom, are visiting him at the new precinct, they drive him to the site of a bank robbery-murder and catch the crooks as they are making their getaway.  Because of publicity about the case, the mayor then decides to name him the new chief of police.  Assuming the office, Jim tells his men that he will be cleaning up the town and expects their complete cooperation.  With Sandy and Mac as his aides, Jim starts closing speakeasies immediately.

That night, depressed because Jim has turned down his request for promotion to captain, Ed goes drinking with Daisy and takes an offer by Belmonte to find a safer route into the city for his "grapefruit" because he needs money to entertain Daisy.

The next day, Jim offers Ed a career-advancing assignment to transport a shipment of money.  Mac and Sandy, who are wary of Ed's relationship with Daisy, then suggest to Jim that they secretly stay in the background to help Ed out if anything goes wrong.  When Ed drunkenly tells Daisy about the job, she tells Cholo, who decides to steal the money; because Daisy pretends that she wants to go away with him, Ed agrees to the plan.

Sandy and Mac, who see the robbery but don't know that Ed is in on it, follow the robbers, the Gorman brothers.  During the chase, a child is killed by the thieves and Mac dies of a bullet wound.  When the Gormans are brought in for questioning, one of the brothers confesses to a shocked Jim that Ed was in on it because he thinks that his brother is being beaten by policemen in another room.  All three are then tried for robbery and murder, but Michaels gets them off by frightening witnesses and charging that the Gormans confessed only to stop being beaten.  When the men are aquitted, the judge admonishes the obviously intimidated jury for their cowardliness.

That night, while Jim despairs, Ed goes to him to beg forgiveness and offers to go to the newspapers with the truth.  Jim tells Ed to go to Belmonte's club and confront him at exactly 3:30 when Jim and his men will be right behind him.  At the club, after Ed confronts Belmonte, Cholo shoots Ed, and a gun battle ensues between the police and Belmonte's men.  Many men on both sides are killed, including Ed, Sandy and Belmonte, who accidentally shoots Daisy.  As Jim falls to the floor, mortally wounded, he reaches for the hand of his brother.