To Kill a Mockingbird takes place in Alabama during the Depression, and is narrated by the main character, a little girl named Jean Louise "Scout" Finch. Her father, Atticus Finch, is a lawyer with high moral standards. Scout, her brother Jem, and their friend Dill are intrigued by the local rumors about a man named Boo Radley, who lives in their neighborhood but never leaves his house. Legend has it that he once stabbed his father in the leg with a pair of scissors, and he is made out to be a kind of monster. Dill is from Mississippi but spends his summer in Maycomb at a house near the Finch's.
The children are curious to know more about Boo, and during one summer create a mini-drama they enact daily, which tells the events of his life as they know them. Slowly, the children begin moving closer to the Radley house, which is said to be haunted. They try leaving notes for Boo on his windowsill with a fishing pole, but are caught by Atticus, who firmly reprimands them for making fun of a sad man's life. Next, the children try sneaking over to the house at night and looking through its windows. Boo's brother, Nathan Radley, who lives in the house, thinks he hears a prowler and fires his gun. The children run away, but Jem loses his pants in a fence. When he returns in the middle of the night to get them back, they have been neatly folded and the tear from the fence roughly sewn up.
Other mysterious things happen to the Finch children. A certain tree near the Radley house has a hole in which little presents are often left for them, such as pennies, chewing gum, and soap carved figures of a little boy and girl who bear a striking resemblance to Scout and Jem. The children don't know where these gifts are coming from, and when they go to leave a note for the mystery giver, they find that Boo's brother has plugged up the hole with cement. The next winter brings unexpected cold and snow, and Miss Maudie's house catches on fire. While Jem and Scout, shivering, watch the blaze from near the Radley house, someone puts a blanket around Scout without her realizing it. Not until she returns home and Atticus asks her where the blanket came from does she realize that Boo Radley must have put it around her while she was entranced by watching Miss Maudie, her favorite neighbor, and her burning house.
Atticus decides to take on a case involving a black man named Tom Robinson who has been accused of raping a very poor white girl named Mayella Ewell, a member of the notorious Ewell family, who belong to the layer of Maycomb society that people refer to as "trash." The Finch family faces harsh criticism in the heavily racist Maycomb because of Atticus's decision to defend Tom. But, Atticus insists on going through with the case because his conscience could not let him do otherwise. He knows Tom is innocent, and also that he has almost no chance at being acquitted, because the white jury will never believe a black man over a white woman. Despite this, Atticus wants to reveal the truth to his fellow townspeople, expose their bigotry, and encourage them to imagine the possibility of racial equality.
Because Atticus is defending a black man, Scout and Jem find themselves whispered at and taunted, and have trouble keeping their tempers. At a family Christmas gathering, Scout beats up her cloying relative Francis when he accuses Atticus of ruining the family name by being a "nigger-lover". Jem cuts off the tops of an old neighbor's flower bushes after she derides Atticus, and as punishment, has to read out loud to her every day. Jem does not realize until after she dies that he is helping her break her morphine addiction. When revealing this to Jem and Scout, Atticus holds this old woman up as an example of true courage: the will to keep fighting even when you know you can't win.
The time for the trial draws closer, and Atticus's sister Alexandra comes to stay with the family. She is proper and old-fashioned and wants to shape Scout into the model of the Southern feminine ideal, much to Scout's resentment. Dill runs away from his home, where his mother and new father don't seem interested in him, and stays in Maycomb for the summer of Tom's trial. The night before the trial, Tom is moved into the county jail, and Atticus, fearing a possible lynching, stands guard outside the jail door all night. Jem is concerned about him, and the three children sneak into town to find him. A group of men arrive ready to cause some violence to Tom, and threaten Atticus in the process. At first Jem, Scout and Dill stand aside, but when she senses true danger, Scout runs out and begins to speak to one of the men, the father of one of her classmates in school. Her innocence brings the crowd out of their mob mentality, and they leave.
The trial pits the evidence of the white Ewell family against Tom's evidence. According to the Ewells, Mayella asked Tom to do some work for her while her father was out, and Tom came into their house and forcibly beat and raped Mayella until her father appeared and scared him away. Tom's version is that Mayella invited him inside, then threw her arms around him and began to kiss him. Tom tried to push her away. When Bob Ewell arrived, he flew into a rage and beat her, while Tom ran away in fright. According to the sheriff's testimony, Mayella's bruises were on the right side of her face, which means she was most likely punched with a left hand. Tom Robinson's left arm is useless due to an old accident, whereas Mr. Ewell leads with his left. Given the evidence of reasonable doubt, Tom should go free, but after hours of deliberation, the jury pronounces him guilty. Scout, Jem and Dill sneak into the courthouse to see the trial and sit in the balcony with Maycomb's black population. They are stunned at the verdict because to them, the evidence was so clearly in Tom's favor.
Though the verdict is unfortunate, Atticus feels some satisfaction that the jury took so long deciding. Usually, the decision would be made in minutes, because a black man's word would not be trusted. Atticus is hoping for an appeal, but unfortunately Tom tries to escape from his prison and is shot to death in the process. Jem has trouble handling the results of the trial, feeling that his trust in the goodness and rationality of humanity has been betrayed.
Meanwhile, Mr. Ewell threatens Atticus and other people connected with the trial because he feels he was humiliated. He gets his revenge one night while Jem and Scout are walking home from the Halloween play at their school. He follows them home in the dark, then runs at them and attempts to kill them with a large kitchen knife. Jem breaks his arm, and Scout, who is wearing a confining ham shaped wire costume and cannot see what is going on, is helpless throughout the attack. The elusive Boo Radley stabs Mr. Ewell and saves the children. Finally, Scout has a chance to meet the shy and nervous Boo. At the end of this fateful night, the sheriff declares that Mr. Ewell fell on his own knife so Boo, the hero of the situation, won't have to be tried for murder. Scout walks Boo home and imagines how he has viewed the town and observed her, Jem and Dill over the years from inside his home. Boo goes inside, closes the door, and she never sees him again.