The story is set in the times of the Great Depression, during 1933-1935, in the fictional little town of Maycomb, Alabama. The protagonist of the story is a girl aged six, Jean Louise Finch (also called Scout), living with her family, who are Scout’s elder brother Jeremy (Jem) and their father, Atticus, a widowed lawyer. The children are friends with the neighborhood boy Dill, and the three spend much time together. There is also a mysterious reclusive neighbor Arthur Radley, nicknamed “Boo”.
While little is known about him, the children are both interested in and afraid of Boo, collecting rumors and making up stories about him and sharing the ideas of how to lure him from his house. Later they start finding small gifts in a tree that Radley leaves for them, but they fail to see him.
The book appears to be divided into two parts, and the second part tells about Atticus being appointed a lawyer to defend Tom Robinson, a Negro man accused of raping Mayella Ewell, white, a daughter of the town drunk. Both Atticus’s relatives and the majority of the town residents think Tom is guilty and should be put to death, disapproving of Atticus’s firm decision to defend him. Atticus is mocked at by some of the town’s dwellers and called the “nigger-lover”. His conservative family members seem ashamed of his actions.
A mob of aggressive men want to lynch Robison, but Scout, Jem and Dill manage to persuade them not to do it.
Although Atticus Finch is against the children attending the trial, they manage to sneak into the courtroom and observe the proceedings from the so-called “coloured balcony” for black people. In course of the trial, Atticus provides evidence that the people accusing Tom – Mayella Ewell and her father Bob Ewell, known as the town drunk – are telling lies. However, despite all the evidence presented to support Robinson’s innocence, the jury, comprised of white farmers, finds him guilty.
When desperate Tom tries to get over the prison fence to escape, he gets shot dead. This shocks Jem and makes both the children and Atticus doubt their faith in justice.
Despite the fact that Bob Ewell has actually won the case, he feels that his credibility is destroyed because of Atticus’s actions. Ewell is enraged and fights back. He spits in Atticus’ face, threatens Robinson’s widow, attempts to break into Judge Taylor’s house, and then attacks Jem and Scout on their way home from the school Halloween celebration.
Scout’s costume protects her from injuries, but Jem’s arm is broken by the attacker. During the fight, an unknown man appears and saves the children. Having defeated their attacker, he carries the wounded Jem home, and Scout understands this man is Arthur “Boo” Radley.
The local sheriff arrives and finds Bob killed in the fight, stabbed with a knife. The sheriff and Atticus finally agree that Bob accidentally killed himself by falling upon his knife.
Scout and Boo walk to his house, and then say goodbye to each other and part, and Scout returns home, thinking about how Boo views life. She and her father stay for a while in Jem’s room, and then she goes to sleep.
The central conflict of the novel deals with racism, prejudice, justice and the lengths a person may go to defend it. The book highlights a situation when the lawyer choosing to stand for an innocent but generally despised black man has to oppose the opinion of the entire town and risk his reputation. His children are also involved and have to suffer for their father’s position, but they are learning from the conflict and have to make tough choices and adopt more mature views of life.