It's 1949 Los Angeles and former boxer turned celebrity mobster Mickey Cohen (SEAN PENN) essentially runs the city of Los Angeles, not only through his own goons -- including Jack Whalen (SULLIVAN STAPLETON)-- but also the cops, judges and others under his control. Chief Parker (NICK NOLTE) of the LAPD is one of the few not in his pocket and he wants the mobster shut down.
Accordingly, he enlists the aid of LAPD cop Sgt. John O'Mara (JOSH BROLIN) -- a WWII vet who doesn't always play by the book -- to draft a small team of lawmen to work undercover to undermine Cohen's various rackets that include drugs, prostitution and, if he has his way, all of the betting action west of Chicago.
While John's pregnant wife, Connie (MIREILLE ENOS), isn't thrilled at all by the idea, she decides if he's going to do that, she's going to help him pick his team. Among them are veteran gunslinger cop Max Kennard (ROBERT PATRICK) and his new associate, Officer Navidad Ramirez (MICHAEL PEŅA); brainiac and family man Officer Conwell Keeler (GIOVANNI RIBISI); Officer Coleman Harris (ANTHONY MACKIE) who's keen on stopping the heroin trade in his neighborhood; and fellow WWII vet Sgt. Jerry Wooters (RYAN GOSLING).
Jerry initially isn't interested, and mainly uses his badge to get him into nightclubs and such. He'd also rather spend time with Grace Faraday (EMMA STONE), a former aspiring actress who just so happens to be Cohen's doll, something their mutual friend Jack thinks is nothing short of playing with fire.
But when a mob hit accidentally includes his favorite shoeshine boy, Jerry joins the team and they begin their campaign of upsetting Cohen's various ventures. The mobster initially thinks that's the work of rivals, but when he eventually figures out it's cops, their lives along with Grace's immediately face increased peril.