It's 1949, and Los Angeles is in the iron grip of notorious mobster Mickey Cohen (Sean Penn). Sgt. John O'Mara (Josh Brolin) is tasked with putting together a special team to bring down Cohen's empire by any means necessary. He's joined by smooth ladies' man Jerry Wooters (Ryan Gosling), Coleman Harris (Anthony Mackie), crack shot Max Kennard (Robert Patrick), Navidad Ramirez (Michael Peña) and wiretapper Conway Keeler (Giovanni Ribisi).
The six begin to violently take down Mickey's operations by wiretapping his home, while Jerry romances gangster moll Grace Faraday (Emma Stone).
Eventually, Cohen figures out their identities; he has Keeler killed and shoots up O'Mara's home, while the Squad barely survives a hit on them in Chinatown.
The five remaining men face off against Mickey at the Park Plaza Hotel. Kennard dies in the gun battle.
O'Mara confronts and defeats Mickey in a bloody fist fight. Cohen goes to prison after Grace testifies in court that she witnessed him killing a man in cold blood.
Jerry and Grace stay together; Ramirez and Harris become beat cops, and O'Mara retires from the police force to live in peace with his wife and newborn son.