To Kill a Mockingbird is primarily a novel about growing up under extraordinary circumstances in the 1930s in the Southern United States. The story covers a span of three years, during which the main characters undergo significant changes. Scout Finch lives with her brother Jem and their father Atticus in the fictitious town of Maycomb, Alabama. Maycomb is a small, close-knit town, and every family has its social station depending on where they live, who their parents are, and how long their ancestors have lived in Maycomb.

A widower, Atticus raises his children by himself, with the help of kindly neighbors and a black housekeeper named Calpurnia. Scout and Jem almost instinctively understand the complexities and machinations of their neighborhood and town. The only neighbor who puzzles them is the mysterious Arthur Radley, nicknamed Boo, who never comes outside. When Dill, another neighbor's nephew, starts spending summers in Maycomb, the three children begin an obsessive — and sometimes perilous — quest to lure Boo outside.

Scout is a tomboy who prefers the company of boys and generally solves her differences with her fists. She tries to make sense of a world that demands that she act like a lady, a brother who criticizes her for acting like a girl, and a father who accepts her just as she is. Scout hates school, gaining her most valuable education on her own street and from her father.

Not quite midway through the story, Scout and Jem discover that their father is going to represent a black man named Tom Robinson, who is accused of raping and beating a white woman. Suddenly, Scout and Jem have to tolerate a barrage of racial slurs and insults because of Atticus' role in the trial. During this time, Scout has a very difficult time restraining from physically fighting with other children, a tendency that gets her in trouble with her Aunt Alexandra and Uncle Jack. Even Jem, the older and more levelheaded of the two, loses his temper a time or two. After responding to a neighbor's (Mrs. Dubose) verbal attack by destroying her plants, Jem is sentenced to read to her every day after school for one month. Ultimately, Scout and Jem learn a powerful lesson about bravery from this woman. As the trial draws nearer, Aunt Alexandra comes to live with them under the guise of providing a feminine influence for Scout.