A police detective pretends to join racketeers in order to clean up organized crime; but he is mistrusted by one of the gangsters.
Nick Fenner (Humphrey Bogart) and Al Kruger (Barton MacLane) see a film about racketeering crime with Kruger's acquittal and the pledge of producer Bryant (Henry O'Neill) to defy threats. Kruger tells Fenner to leave Bryant alone; but Bryant is shot dead. Johnny Blake (Edward G. Robinson) and Herman (Frank McHugh) see the news. Herman gives Lee Morgan (Joan Blondell) the money from the numbers collections. Blake calls on Kruger, who offers him a job even though he is a police detective. Kruger meets with bankers, who ask who killed Bryant; they suspect Fenner and warn Kruger. Police Captain Dan MacLaren (Don King) agrees to head a commission on corruption. Blake gets fired. Lee gets $4,000 from Nellie (Louise Beavers) from numbers in Harlem. Lee calls on Blake and asks him to run the numbers game, but he says no. At a boxing match Blake punches MacLaren and is thrown out.
Kruger asks Blake to find the weak spots in his organization. Blake agrees, but Fenner tells Kruger to keep Blake away from his milk and produce. Kruger takes Blake to the garage, where cash is collected, and records are burned. Blake meets a wire-tapper now working for Fenner. Blake finds a microphone in a lamp and fires his gun. MacLaren makes arrests, and Fenner thinks Blake is tipping off the police. MacLaren stops Fenner's produce business. Blake fights a policeman and is thrown in jail, where MacLaren says Fenner is after him. Blake wants to replace Fenner and tells MacLaren to keep smashing them. Kruger's lawyer gets Blake out. Blake thanks Kruger and is called a stool pigeon. Blake says they need help and suggests new rackets like the numbers game.
Lee and Nellie are told to retire, and Herman says men took the money. Blake objects to Fenner taking over the numbers game. Kruger says it is Blake's. Fenner tells Lee that Blake is taking over numbers; she asks Blake and feels betrayed. Blake collects money. Fenner shoots Kruger and offers Lee the numbers in the Bronx and Harlem. He kisses her; but she slaps him. Fenner takes over for Kruger; but Blake says the bosses gave him the job with orders to deliver money. The bankers calls in Blake, give him the job, and ask for the money. Blake picks up cash and sends in the cops. Fenner escapes. Lee passes on Blake's address to Fenner, who shoots Blake. Blake is wounded and kills Fenner. Lee picks up Blake and drives him to the bankers. Blake gives them the money and collapses in the doorway. The police arrest the bankers, and the dying Blake tells MacLaren that Fenner killed Bryant and Kruger.
This drama explores racketeering and how police can locate it by infiltration. Kruger trusts Blake because he keeps his word, while, ironically, Blake is deceiving him.