A backwoods politician rises to the top only to become corrupted.

Reporter Jack Burden is sent to cover the campaign of Willie Stark, a small-town Southern reform candidate for county treasurer. Willie is determined to expose government corruption, and those in power are equally determined that he will not be allowed to run. After Willie is released from jail, having been arrested for illegally holding a political rally, he takes Jack home to meet his father, his wife Lucy, a former schoolteacher, and their teenaged, adopted son Tom. Jack is impressed by the Starks's fortitude and, convinced of Willie's honesty, writes a series of favorable articles about him. On a trip home to Burden's Landing, Jack's wealthy stepfather, Floyd McEvoy, scoffs at the idea that Willie is honest and unbuyable. Later, Jack proposes marriage to Anne Stanton, the niece of their neighbor, Judge Stanton, and sister of his best friend, Adam, but she turns him down. Willie loses the election but later, with Lucy's tutoring, acquires a law degree. Sometime later, several grade-school children are killed when a fire escape they are using collapses during a fire drill, and Willie files a damage suit against the builders. The voters, remembering his charge that the building contract went to a relative of the county commissioner, are now willing to support Willie. Threatened by a serious reform movement, corrupt state politicians convince Willie to run for governor, thinking that he will split the reform vote and allow their candidate to win. Jack travels with the campaign but, like the voters, is unimpressed by Willie's dull, fact-filled speeches. One evening, while Jack works with Willie to improve his oratorical skills, Sadie Burke, who has been sent on the campaign by the political machine to keep an eye on Willie, tells him the truth--that he is a decoy. The next day, in the best speech of his career, a hungover Willie presents the facts to his audience, telling them that they are "hicks" just like him. Although he loses the election, he has learned what it will take to win in the future. Four years later, Willie again runs for governor and hires Jack as his aide. This time, he finances his campaign by making deals with anyone who will help him. On a trip to Burden's Landing with Jack, Willie admits that he has made promises, but argues persuasively that good can come out of evil and thus wins the support of the Stantons, particularly Anne. After he is elected, Willie pushes bills through the legislature, instituting benefits for farmers and allocating money for schools, football stadiums and hospitals. He also starts drinking heavily and pursuing young women, to the disgust of Sadie, who has become his mistress. Willie appoints Stanton to the office of Attorney General, but Stanton resigns when Willie suppresses information about graft in his administration. Willie then assigns Jack to dig up scandal in Stanton's background. Although he is reluctant, Jack is too dependent on Willie to refuse, even after Sadie reveals Willie's ongoing affair with Anne. After a drunken Tom causes the death of Helene Hale during a driving accident, he publicly admits his guilt, despite Willie's attempts to cover up the incident. Willie then offers Helene's father a bribe, which is refused. Later, Willie insists that Tom play in a football game, and Tom ends up paralyzed after an injury sustained in the accident is exacerbated. When Adam refuses to head Willie's new medical center, Jack shows Anne the evidence he has uncovered about the judge and offers to keep it a secret if Adam takes the job. Then Hale is killed, and Stanton accuses Willie of murder. As impeachment proceedings begin, Willie demands that Jack disclose what he has discovered about Stanton. Privately, Jack begs Stanton to stop the proceedings, and is stunned when Willie reveals that he already knows about the judge's background. Stanton commits suicide, and Jack, suspecting that Anne was the source of Willie's information, quits his job in disgust. When Willie is not impeached, Adam shoots him during a victory speech and is killed himself by the police. After Jack unsuccessfully tries to convince Anne to reveal the truth about Willie to the crowd, Willie dies in Jack's arms, without understanding why he has been killed.