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 neighbours and a black housekeeper named Calpurnia. Scout and Jem almost instinctively understand the complexities and machinations of their neighbourhood and town. The only neighbour who puzzles them is the mysterious Arthur Radley, nicknamed Boo, who never comes outside. When Dill, another neighbour'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 criticises 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.
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 level-headed of the two, loses his temper a time or two. After responding to a neighbour'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.
Tom is tried and convicted even though Atticus proves that Tom could not have possibly committed the crime of which he is accused. In the process of presenting Tom's case, Atticus inadvertently insults and offends Bob Ewell, a nasty, lazy drunkard whose daughter is Tom's accuser. In spite of Tom's conviction, Ewell vows revenge on Atticus and the judge for besmirching his already tarnished name. All three children are bewildered by the jury's decision to convict. Atticus tries to explain why the jury's decision was in many ways a foregone conclusion.
Shortly after the trial, Scout attends one of her aunt's Missionary Society meetings. Atticus interrupts the meeting to report that Tom Robinson had been killed in an escape attempt. Scout learns valuable lessons about achieving the ideal of womanhood and carrying on in the face of adversity that day.
Things slowly return to normal in Maycomb and Scout and Jem realise that Boo Radley is no longer an all-consuming curiosity. The story appears to be winding down, but then Bob Ewell starts making good on his threats of revenge. Scout is in the Halloween pageant at school, playing the part of a ham. With Atticus and Aunt Alexandra both too tired to attend, Jem agrees to take Scout to the school. After embarrassing herself on-stage, Scout elects to leave her ham costume on for the walk home with Jem.
On the way home, the children hear odd noises, but convince themselves that the noises are coming from another friend who scared them on their way to school that evening. Suddenly, a scuffle occurs. Scout really cannot see outside of her costume, but she hears Jem being pushed away and feels powerful arms squeezing her costume's chicken wire against her skin. During this attack, Jem badly breaks his arm. Scout gets just enough of a glimpse out of her costume to see a stranger carrying Jem back to their house.
The sheriff arrives at the Finch house to announce that Bob Ewell has been found dead under the tree where the children were attacked, having fallen on his own knife. By this time, Scout realises that the stranger is none other than Boo Radley and that Boo is actually responsible for killing Ewell, thus saving her and Jem's lives. In spite of Atticus' insistence to the contrary, the sheriff refuses to press charges against Boo. Scout agrees with this decision and explains her understanding to her father. Boo sees Jem one more time and then asks Scout to take him home, but rather than escort him home as though he were a child, she has Boo escort her to his house as a gentleman would.
With Boo safely home, Scout returns to Jem's room where Atticus is waiting. He reads her to sleep and then waits by Jem's bedside for his son to wake up.