Scout Finch, the narrator, lives with her brother Jem, and their widowed father Atticus, in the Southern stagnant town of Maycomb, Alabama.
Though already suffering beforehand, Maycomb had been hit with the Great Depression in the 1930's. Atticus is a prominent lawyer of the time and the Finch family is reasonably well-off in contrast to the rest of the community.
One summer, Jem and Scout meet a young boy named Dill, who they quickly befriend. He has come to live in their neighbourhood for the summer, and in this time the three children act out stories together.
Dill soon becomes very interested in a house on their street. This house is owned by Mr Nathan Radley, whose reclusive brother, Arthur (known as Boo by the children), has lived there for years without being seen outdoors.
Much against her will, Scout begins school. As she and Jem walk past the Radley place to and from school, they find gifts seemingly left for them in the knothole of a tree on the Radley property.
The following summer when Dill returns, he, Scout, and Jem begin to act out the story of Boo Radley which was built upon rumours and just plain imagination. Soon enough Atticus finds them and stops them, urging the children to attempt to view life from another person’s perspective before making judgments.
On Dill’s last night of his time in Maycomb for the summer, the children sneak onto the Radley property in order to try and catch a glimpse of Boo through a window. Startlingly, Nathan Radley shoots at them, convinced it was a negro in his garden. In trying to escape back under the wire fence, Jem loses his pants. He later returns for them, and he finds them mended and hung over the fence.
The next winter, Jem and Scout continue to find presents in the tree's knothole, and they now presume they are left by the mysterious Boo. Unfortunately for the children, Mr Radley plugs the knothole with cement.
Not long after, a fire breaks out in neighbour Miss Maudie's house. As Scout stands on the street with Jem, watching the blaze, she discovers that a blanket has been slipped over her. Convinced that Boo was the one who covered Scout with a blanket, Jem tells Atticus about the mended pants and the gifts in the tree. The children develop confusion over the acts of kindness from Boo, who they initially believed to be a monster.
Atticus is chosen to defend negro man Tom Robinson, who has been accused of raping a white woman - daughter of Bob Ewell. In this decision, Jem and Scout are victims of abuse from other children whose parents are racially prejudice. Calpurnia, the Finches’ negro housekeeper, takes them to the local black church, where the black community embraces the children.
Aunt Alexandra, Atticus's strong-willed sister comes to live with the Finches the next summer. Dill also runs away from where he was staying and returns to Maycomb. The trial of Tom Robinson begins, and he is placed in a local jail cell, where Atticus stays overnight to protect Tom. That night mob of men from around town arrives to lynch him. Jem and Scout, who have sneaked out of the house, soon join Atticus to face the mob. As Scout recognizes one of the men as Mr Cunningham, she politely questions him about his son which results in him dispersing the mob in his shame.
The children also sneak out of the house to watch the trial. They sit up in the balcony with Maycomb's black citizens.
During the trial, Atticus provides clear evidence that the accusation of rape is in fact a lie. Mayella Ewell invited Tom Robinson in to the house before being caught by her father. Mayella then accused Tom of rape, in order to cover her shame and guilt for relations with a negro.
Despite the significant evidence pointing to Tom’s innocence, the jury, who are all white men, convicts him. After watching the trial, Jem's faith in the system of justice is shaken. Tom Robinson later tries to escape from prison and is shot to death.
After all this, Bob Ewell vows revenge as he feels that Atticus has made a fool out of him. Ewell abuses Tom Robinson’s widow, and attacks Jem and Scout as they walk home from a function. After a struggle between Jem and Bob Ewell, Boo Radley intervenes and saves the children. Boo then carries the wounded Jem back to the Finch house. It is discovered that Ewell was stabbed by a kitchen knife in the struggle, and in order to protect Boo, the local sheriff insists that Ewell fell on his own knife, killing himself. Scout and Boo sit together for a while, before Boo returns to the Radley house.
Scout feels that she can finally imagine what it would be like to be Boo, and takes her father's advise given to her earlier on being sympathetic. She now see's that Boo Radley is just like a mockingbird.