It's the 1950s and Willie Stark (SEAN PENN) is an unpolished but passionate minor politician who's most concerned with the local corruption that led to several children perishing from shoddy school construction. His actions are soon noticed not only by local newspaper reporter Jack Burden (JUDE LAW), but also political operative Tiny Duffy (JAMES GANDOLFINI) who dupes Willie into running for governor, all for his handlers who think they can control the neophyte. But when Willie gets wind of this from Jack and his own press attaché Sadie Burke (PATRICIA CLARKSON), he discards his prepared speech and speaks from his heart.
His demagoguery soon attracts growing masses of the disenchanted masses, which doesn't sit well with the rich or corporations who fear what his idealistic desires might mean for them. It's not long before the recently fired Jack becomes his right-hand man and follows him to the governor's mansion when Willie is elected. With him, Sadie, Tiny and bodyguard Sugar Boy (JACKIE EARLE HALEY) making up his core team, he sets out to change Louisiana politics, thus drawing the ire of retired but still highly influential Judge Samuel Irwin (ANTHONY HOPKINS) who calls for his impeachment.
Inexorably becoming seduced by the power of his seat, Willie assigns Jack to dig up whatever dirt he can find on Irwin -- who just so happens to be Jack's godfather -- in hopes of persuading him to change his stance. As Jack tries to do just that, he's reunited with his former childhood friends, siblings Anne (KATE WINSLET) and Adam Stanton (MARK RUFFALO), the now adult children of a former governor. As Jack's romantic feelings for Anne are rekindled and they discuss how Adam, a doctor, has changed, the balance of power in Louisiana politics remains in the balance.