In the town of Maycomb, Alabama, in the middle of the Great Depression, six-year-old Scout Finch lives with her older brother Jem, and her widowed father, Atticus. Atticus is a lawyer, and makes enough to keep the family comfortably out of poverty, but he works long days. He relies on the family's black cook, Calpurnia, to help raise the kids.
Scout and Jem spend much of their time creating and acting out fantasies. One year a boy named Dill comes to spend the summer with his aunt, who is the Finches' neighbor. The three children become friends, and, pushed by Dill's wild imagination, soon become obsessed with a nearby house. A man named Nathan Radley owns the house, but it is his reclusive brother, Arthur (Boo) Radley, who interests and terrifies the children. On a dare, Jem actually runs up and touches the Radley house, and Scout is sure she sees someone watching them from inside behind a curtain.
The summer ends, and Dill returns to his hometown. Scout starts school, and hates it. On the first day, her teacher actually criticizes her for already knowing how to read. The highlights of the school year come when Scout and Jem occasionally find presents stuffed into a knothole of a tree next to the Radley's fence. Summer arrives, and Dill returns. The children grow more daring and sneak onto the Radley property. But Nathan Radley sees them and thinks they're thieves. They run, and Jem's pants get caught in the Radley fence. He leaves them behind. When he goes back to get them later that night, the pants are mended and folded. Meanwhile, Scout and Jem continue to find gifts in the knothole until Nathan Radley cements it shut. A few months later, in the dead of winter, the Finch's neighbor Miss Maudie Atkinson's house catches fire, and as Scout and Finch watch it burn someone Scout doesn't see puts a blanket around her shoulders. Jem realizes that Boo Radley must have done it.
That year, Atticus is appointed by the court to defend a black man, Tom Robinson, who is accused of raping Mayella Ewell, the daughter of a poor, notoriously vicious white man named Bob Ewell. Racial tensions in Maycomb flare. Scout and Jem become targets of abuse from schoolmates, neighbors, townspeople, and even some family members. In contrast, when Calpurnia takes the children to attend her black church, they are for the most part warmly received.
Before the trial starts, Atticus' sister Alexandra comes to live with the Finch's. Dill also arrives, after sneaking away from his mother and her new husband. Alexandra's social views are more conservative than Atticus's. She treats Calpurnia more like a servant than a family member and tries to make Scout act more like a girl. The day before the trial, a mob surrounds the jail where Tom Robinson is being held. Scout, Jem, and Dill, who have snuck out of their house, join Atticus, who anticipated the mob attack. Scout doesn't realize what's going on, but recognizes a man in the crowd and asks him about his son, who is Scout's classmate. The man, shamed, disperses the mob.
At the trial, Atticus presents a powerful defense of Tom and makes it clear that Ewell is lying. The children sneak into the trial and watch the proceedings from the balcony, where the black people are forced to sit. Jem is sure Atticus will win the case, but the all-white jury still convicts Tom. Jem is particularly hard hit by the verdict, and his faith in justice is even further shaken when Tom tries to escape from prison and is shot and killed.
Even though Robinson was convicted, Ewell is furious that Atticus made him look like a fool. One night, as Jem and Scout walk home alone from a Halloween pageant, Ewell attacks them. Jem's arm is broken, but someone rushes in to help. In the scuffle, Ewell is killed. The man who saved Jem and Scout carries Jem home, and Scout realizes that the man is Boo Radley. Heck Tate decides to keep Radley's involvement in Ewell's death quiet, and Scout walks Radley home. As Scout stands on the Radley porch, she sees the world as Boo must see it. When she gets home, Scout falls asleep as Atticus reads to her at Jem's bedside.