Scout Finch lives in a quiet town called Maycomb, Alabama, with her brother, Jem, and their father, Atticus, who is a lawyer. At this time, many people are struggling because of the Great Depression, but the Finch family is doing better than most. One summer, they meet a boy named Dill, who comes to stay in their neighborhood. The three of them become great friends and enjoy acting out stories together. Dill becomes very curious about a mysterious house on their street known as the Radley Place. The house belongs to Mr. Nathan Radley, and his brother, Boo Radley, has not been seen outside for many years.

When school starts that fall, Scout is not happy at all. She and Jem discover gifts left for them in a knothole of a tree on the Radley property. Dill returns the next summer, and the three friends begin to act out the story of Boo Radley. Atticus advises them to think about things from other people's points of view before judging them. One night, on Dill’s last evening in Maycomb, they sneak onto the Radley property and are scared when Nathan Radley shoots at them. In the chaos, Jem loses his pants, but when he goes back to find them, he sees that they have been fixed and hung over the fence.

The following winter, Jem and Scout find more gifts in the tree, which they believe are from Boo. However, Nathan Radley fills the knothole with cement, so they can no longer receive gifts. Soon after, a fire breaks out in a neighbor's house. As they watch the fire, someone drapes a blanket around Scout's shoulders. She is sure that Boo must have done it. Jem tells Atticus about the mended pants and the gifts they found.

In the town of Maycomb, Atticus decides to defend a Black man named Tom Robinson, who is accused of a crime against a white woman. This decision makes some people in the town very angry, and Jem and Scout face bullying from other children, even during Christmas celebrations. Their housekeeper, Calpurnia, takes them to a Black church, where they are welcomed warmly by the community.

The next summer, Aunt Alexandra comes to live with them. Dill, who was supposed to stay with his father, runs away and comes to Maycomb. The trial of Tom Robinson begins, and a mob gathers to harm him. Atticus bravely stands against the mob, and Jem and Scout secretly join him. Scout recognizes one of the men and talks to him about his son, which makes him feel ashamed and causes the mob to leave.

During the trial, the children sit in the section for Black citizens. Atticus shows that the people accusing Tom, Mayella Ewell and her father, Bob Ewell, are lying. He explains that Mayella tried to flirt with Tom, and when her father found them together, he beat her. Despite the strong evidence pointing to Tom's innocence, the all-white jury finds him guilty. Later, Tom tries to escape from prison and is shot and killed. This outcome deeply disappoints Jem, who starts to lose faith in justice.

After the trial, Bob Ewell feels humiliated and seeks revenge. He threatens Tom's widow, tries to break into the judge's house, and finally attacks Jem and Scout on their way home from a Halloween party. Boo Radley comes to their rescue, saving the children and fatally wounding Ewell. Boo carries the injured Jem back to their home, and to protect Boo, the sheriff claims that Ewell fell on his own knife.

Later, as Scout reflects on everything that happened, she starts to understand Boo better. He becomes a real person to her. With this understanding, Scout learns to see things from others' perspectives and realizes that even in a world full of prejudice, there is still goodness in people.