In the small town of Clanton, in Ford County, Mississippi, a ten-year-old African-American girl named Tonya Hailey is viciously raped and beaten by two racist white supremacists, James Louis "Pete" Willard and Billy Ray Cobb. Tonya is later found and rushed to a hospital while Pete and Billy Ray are heard bragging in a roadside bar about their crime. Tonya's distraught and outraged father, Carl Lee Hailey, consults his friend Jake Brigance, a white attorney who had previously represented Hailey's brother, on whether he could get Carl Lee acquitted if he killed the two men. Jake tells Carl Lee not to do anything stupid, but admits that if it had been his daughter, he would kill the rapists. Carl Lee is determined to avenge Tonya and, while Pete and Billy Ray are being led into holding from their bond hearing, he kills both men with an M-16.
Carl Lee is charged with capital murder. Despite efforts to persuade Carl Lee to retain high-powered attorneys, he elects to be represented by Jake Brigance. Helping Jake are two loyal friends, disbarred attorney Lucien Wilbanks and sleazy divorce lawyer Harry Rex Vonner. Later, the team is assisted by liberal law student Ellen Roark, who has prior experience with death penalty cases and offers her services as a temporary clerk pro bono. Ellen appears to be interested in Jake romantically, but the married Jake resists her overtures. The team also receives some illicit behind-the-scenes help from black county sheriff Ozzie Walls—a figure beloved by the black community and also well respected by the white community—who upholds the law in making the arrest of Carl Lee but, as the father of two daughters of his own, privately supports Carl Lee and gives him special treatment while in jail and goes out of his way to assist Jake and his team in any way he legally can. Carl Lee is prosecuted by Ford County's corrupt district attorney, Rufus Buckley, who hopes that the case will boost his political career. It is claimed that the judge presiding over Carl Lee's trial, Omar "Ichabod" Noose, has been intimidated by local white supremacist elements. This proves true when, despite having no history of racist inclinations in his rulings, Noose refuses Jake's perfectly reasonable request for a change of venue, even though the racial make-up of Ford County virtually guarantees an all-white jury.
Billy Ray's brother, Freddy, seeks revenge against Carl Lee, enlisting the help of the Mississippi branch of the Ku Klux Klan and its Grand Dragon, Stump Sisson. Subsequently, the KKK attempts to plant a bomb beneath Jake's porch, leading him to send his wife and daughter out of town until the trial is over. Later, the KKK attacks Jake's secretary, Ethel Twitty, and kills her frail husband, Bud. On the day the trial begins, a riot erupts between the KKK and the area's black residents outside of the courthouse; Stump is killed by a molotov cocktail. Believing that the black people are at fault for Stump's death, Freddy and the KKK increase their attacks. As a result, the National Guard is called to Clanton to keep the peace during Carl Lee's trial. Undeterred, Freddy continues his efforts to get revenge for Billy Ray's death. The KKK shoots at Jake one morning as he is being escorted into the courthouse, missing Jake but seriously wounding one of the guardsmen assigned to protect him. They continue to burn crosses throughout Clanton. Later, they burn down Jake's house.
Despite the loss of his house and several setbacks at the start of the trial, Jake perseveres. Jake badly discredits the state's psychiatrist by establishing that he has never conceded to the insanity of any defendant in any criminal case in which he has been asked to testify, even when multiple other doctors have been in consensus otherwise. He traps the doctor with a revelation that several previous defendants found insane in their trials are currently under his care despite his having testified to their "sanity" in their respective trials. Jake follows this up with a captivating closing statement. After lengthy deliberations, the jury acquits Carl Lee by reason of temporary insanity. Carl Lee returns to his family, and the story ends with Jake, Lucien, and Harry Rex having a celebratory drink before Jake is to hold a press conference and leave town to reunite with his family.