Since the novel isn't linear, it's easy to get confused at various points. In keeping with the anti-chronological nature of All the King's Men, this summary will do a bit of time traveling, but we'll keep you up to speed on the time period in which the events are taking place.
When we first meet our protagonist, Jack Burden, he is remembering a road trip to Mason City with Willie Stark, Tom Stark (Willie's son), and Lucy Stark (Willie's wife). The year is 1933, three years before the "present" in the novel. After visiting the Stark family home, Willie and Jack go to Mason City to put political pressure on Jack's mentor, Judge Irwin. After the visit, Willie tells Jack to find out what secrets Judge Irwin might have in his past.
Jack remembers the first time he saw Mason City. The year is 1922 and he is trying to find out about Willie so he could write an article about him for the Chronicle. (At this point in time, Jack is a journalist for the newspaper.) He learns that Willie is fighting the Mason City big wigs because they want to give a contract to build the new school to a company that has bid extremely high. As it so happens, the company is connected to political power in the state. Willie loses this battle, but is vindicated when new school's faulty fire escapes built by the corrupt contractor collapse during a fire drill, injuring and killing many children. Willie becomes a Mason County hero for having sniffed out the corruption.
When some politicians convince Willie to run for governor, he has no idea they are doing it to fix the race so one of the other candidates will win. Since Willie's campaign speehes are so uninspiring, he has no chance of winning. When he learns he's just a pawn, he gets drunk for the first time in his life. To help him recover from his hangover, and face the political barbeque he's due to attend, Jack gets him drunk again. This allows the powerful speechmaker inside Willie to emerge for the first time.
Soon enough, Willie runs for governor again, and easily wins. He hires Jack to come and work for him. Meanwhile, Jack remembers a visit to his mother in 1933. We learn that she treats Jack a little strangely, that she's had a long history with men, and that she's wealthy. Driving back from her place, Jack imagines her meeting his father, Ellis Burden, in Arkansas in 1896.
When Jack gets back to the Capitol after visiting his mother, Willie tells Jack he wants to build a new hospital that will be completely free to the public. Basically, the new hospital will be the best of the best. Then Jack learns that in addition to other indiscretions, Willie has been having an affair with Sadie Burke, his secretary. By 1936, Lucy and Willie are no longer living together, but are still married.
We next move from the 1930s back to pre-Civil War times. We find out that while Jack was working toward his Ph.D. in history, he was sent some papers belonging to Cass Mastern, Ellis Burden's uncle. From the papers, Jack learns Cass's amazing story. Let's jump back in time into Cass's story.
As a young man, Cass Mastern learns plantation management from his brother Gilbert, a plantation owner, and is given a small plantation of his own. Cass also begins an affair with Annabelle, the beautiful twenty-nine-year-old wife of his good friend, Duncan Trice. When Duncan discovers the affair, he shoots himself. After the funeral Annabelle becomes convinced that her slave, Phebe, knows the whole sordid tale, and so sells her into sexual slavery. When Cass finds out, he suddenly realizes how horrible slavery is, and he tries to find Phebe.
Cass is unsuccessful in finding Phebe, but he does free all the slaves on his plantation and tries to help them succeed in life, even though this is before slavery was abolished in the U.S. Yet, before Cass accomplishes his dream of abolishing slavery, he dies. Sadly, he passes away feeling that he is a great sinner, both because of his affair with Annabelle and because of his involvement in slavery.
After Jack learns Cass's story, he is so disturbed by it that he walks out on his Ph.D. program. After the story about Cass, Jack tells us how he began his dig for the dirt on Judge Irwin. Since Irwin and Ellis Burden (Jack's father) were best friends at one time, Jack starts his search with Ellis. But, his father doesn't want to talk about the past. However, through lots of digging, Jack learns that Judge Irwin took a bribe that made him rich, but drove another man to suicide. Jack also discovers that then Governor Stanton, father of his best friend Adam and his own true love, Anne, were also implicated in the crime.
Soon, Willie asks Jack to convince Adam to be the director of his new hospital. Since Adam hates Willie, this isn't an easy task. Anne comes to Jack and begs him to help her convince Adam to accept. (She learned of the offer when having lunch with Willie, soliciting funds for the Children's Home.) Jack tells her that the only way to convince Adam is to reveal what he discovered about Stanton and Irwin.
Jack has a specific plan of action that he thinks will convince Adam to take the hospital directorship. Adam believes that Willie is corrupt because he uses blackmail to get what he wants politically. Jack knows that when Adam learns that both of his idols, Judge Irwin and his own father, were engaged in corruption for pure financial gain he will see that Willie is no worse than anyone else. Jack's hope is that Adam will see that he has no valid reason to condemn Willie, and thus no reason to refuse the directorship.
When Jack tells Anne the dirty truth about Governor Stanton (her father) and Judge Irwin, she takes it pretty hard. Anne ultimately decides that the best way to get Adam to take the hospital position is to tell him the truth about their father and Judge Irwin. This plan works, and Adam accepts the directorship. Shortly after, Jack learns that Anne and Willie have been having an affair.
Then Jack drives to California and relives his life in memory. He remembers growing up with Anne and the two of them falling in love, and also breaking off their relationship. He remembers his subsequent marriage to Lois Seager, and his divorce. By the time Jack is ready to go back home, he has developed a philosophy called the Great Twitch (sometimes referred to as the Big Twitch). Jack decides that all of life is just electrical impulse, and that nothing matters in the end. This is a kind of coping mechanism, a way for Jack to continue living despite his unbearable pain and heartbreak.
Back at the Capitol, Jack learns that Willie's son, Tom, might have gotten a girl named Sibyl Frey pregnant. It also turns out that Willie is being blackmailed with this information by Gummy Larson, who wants Willie to give him the hospital contract. Eventually, Willie thinks that the only way to solve the problem is by getting Judge Irwin to tell the blackmailers to lay off. Willie sends Jack to blackmail Irwin into stopping Willie's blackmailers. But rather than do what Jack and Willie ask, Judge Irwin kills himself. Jack learns that Irwin is really his father, and that his mother and Judge Irwin were lovers.
With Irwin dead, Willie gives Gummy and the blackmailers the hospital contract in order to stop them from leaning on Tom. Tom, however, is injured in a football game, and suffers substantial brain damage. He survives but will be permanently brain dead. Willie takes back the hospital contract from Gummy. This leads the blackmailers to put pressure on Willie through Adam Stanton. Gummy calls Adam and tells him that Willie is having an affair with Anne (Adam's sister), and that this was the only reason Adam was asked to be director. Adam believes it and shoots Willie. Sugar-Boy, Willie's driver, however, shoots Adam and kills him. Shortly after Willie dies, Tom also passes away.
After all the tragedy, Anne and Jack get married. They live with Ellis Burden, who is very sick. Now that he knows the truth about his past, Jack finds hope. He tells us that he is going to write a book about Cass Mastern, and then leave town with Anne.