The Finch children, Jem and Jean Louise (known as Scout) live in Maycomb, Alabama, where they are brought up during the Great Depression by their widower father, Atticus, with the help of their housekeeper Calpurnia. The children meet Dill from Meridian, who is staying in their town for the summer. The three friends become fascinated and frightened by the story of Boo Radley, the Finches' neighbour, who has been locked in his house for years; they repeat the town's scary stories about him to each other, and try to entice him out, without success. When they go back to school, Jem and Scout discover gifts hidden for them in a hole in a tree.

Atticus, a lawyer, is asked to defend a black man, Tom Robinson. He has been accused of raping a white woman, Mayella Ewell, daughter of a drunk who lives next to the town dump. Tom Robinson is innocent: he is a moral, decent man with a disability that means that he clearly couldn't have attacked Mayella. But due to the intense racism of the time, Tom is already thought guilty, and a mob tries to lynch him – only backing off when Atticus, with some help from the children, blocks them.

At the trial, the jury find Tom Robinson guilty.  Jem, Scout and Dill watch the trial from the 'colored' balcony and are dismayed by the injustice of the event, but their father points out that the jury took a long time to decide against Tom Robinson, and that this represents progress. He is reasonably hopeful about their chances on appeal.

However, in prison, Tom feels hopeless, and is terrified of the death penalty. He tries to escape, but his disability defeats him and he is shot repeatedly.

Though Mayella has won the case, her father Bob Ewell is angry at being shamed by Atticus in court, where he was portrayed as 'white trash'. He swears revenge, spitting in Atticus's face, harassing Tom Robinson's widow and breaking into the home of the judge who heard his case.

His anger culminates in him drunkenly attempting to murder Jem and Scout, who are returning from a school pageant. Scout is wearing a chickenwire costume which saves her from Bob's blows as he tries to stab the children. During the brawl, Bob breaks Jem's arm. The children are fighting a losing battle until someone intervenes to save them. They are taken home, where their saviour is found to be Boo Radley. Bob Ewell is found dead at the scene of the fight, with a knife in him; Atticus and the town's sheriff debate what happened, and the sheriff concludes that Bob 'fell on his own knife'. Neither Jem nor Boo Radley will be investigated.

Scout accompanies Boo Radley back to his house, where she realizes that he was nothing to be scared of, that he was the source of the gifts they found in the tree, and that he had been looking out for them as best he could. She regrets that they have never repaid his kindness.  It is the last time she will ever see him.