In the 1940s, former boxing champion Mickey Cohen (Sean Penn) was the most powerful gangster in Los Angeles. He ran prostitution, gambling and other activities while bribing the Los Angeles police and judges, doing evil in Los Angeles and dominating the area. Los Angeles police officer John O'Mara (Josh Brolin) hated evil and had never been afraid of Micky. He had just destroyed one of his prostitution dens. This time, Los Angeles Sheriff Bill Parker decided to fight Micky. He asked John to organize a secret team to eradicate Micky. At the suggestion of his wife, John found his friend Jerry (Ryan Gosling) in the police station, who is also the boyfriend of Micky's lover Grace (Emma Stone); sharpshooter Max Kennard; the equally hateful black policeman Coleman Harris; former intelligence agency officer Conwell Keeler; Max's apprentice, Mexican Navidad Ramirez, and formed the Gangster Squad. They installed wiretaps in Micky's home, disrupted Micky's actions many times, and destroyed his many gambling dens. They thought they had won, but the real battle had just begun...