Scout Finch, almost six years old, her brother Jem, four years older, and their little friend Dill (Charles Baker Harris), a visitor to Maycomb, Alabama, spend their summer thinking of ways to lure Boo Radley from his house. The children never have seen the recluse, but a few townspeople saw him some years ago when Boo reportedly stabbed his father in the leg with a pair of scissors, was locked up for a time, and then was returned to his family. No one in Maycomb has seen him since.
Challenged by Dill, Jem, although fearful he will be killed by Boo—who “dined on raw squirrels and any cats he could catch” —runs and touches the Radley house. The children flee home and look back to see what appears to be an inside shutter move.
In the fall, Scout enters school and gets into trouble in class because she can already read and out of class for fighting with boys. During the year, she and Jem find children’s treasures in a knothole in an oak tree on the Radley place. Before they can put a thank-you note in the tree for the unknown benefactor, Nathan Radley, Boo’s brother, fills the knothole with cement.
The next summer Dill returns. Rolling inside a runaway tire, Scout slams into the Radley porch. She hears laughing inside as she recovers and runs. The three children play Boo Radley games until stopped by Jem and Scout’s father, Atticus.
The last night of Dill’s visit, the three try to look in a window of the Radley home. Jem raises his head to look in, and the children see a shadow coming toward them. They run and a shotgun roars. Jem catches his pants on a wire fence and has to leave them there. After Nathan tells the neighbors he fired at an intruder, Jem goes back for his pants and finds them not only mended but also neatly folded over the fence.
The next winter it snows in Maycomb, and Scout and Jem make their first snowman. During the cold snap, the house of a neighbor, Miss Maudie Atkinson, burns down. Back home after shivering from the cold with the other onlookers, Scout discovers a blanket placed around her shoulders. The only adult in town not at the fire is Boo Radley. Jem tells his father of the treasures in the tree and about his mended pants, fixed by the strange man who never hurts them even when he has the chance.
Scout and Jem begin hearing their father called a “nigger-lover” around town, because of his appointment to defend a black man, Tom Robinson. Atticus warns them to hold their heads high and to not fight about it, but at Christmas Scout bloodies a boy cousin’s nose for repeating the accusation.
The brother and sister receive air rifles for Christmas but are cautioned by their father that to kill a mockingbird is a sin. Their friend Miss Maudie later explains that mockingbirds only make music and sing their hearts out for people.
One day a mad dog comes down the street, and the town’s sheriff asks Atticus to shoot it. He dispatches it with one shot. The children are told that their father, whom they think of as old and feeble, was once known as One-Shot Finch, the best shot in Maycomb County.
An old lady, Mrs. Henry Lafayette Dubose, baits Jem by calling Atticus a “nigger-lover.” Enraged, Jem knocks the tops off her flowers. His father orders Jem to read to the sick woman every afternoon for two months. After her death, Atticus tells the children Mrs. Dubose, although unpleasant, was the bravest woman he ever knew; she broke a morphine habit rather than die addicted. Real courage, the father says, is not a man with a gun in his hand. “It’s when you know you’re licked before you begin but you begin anyway and you see it through no matter what.”
Scout and Jem go to an African American church with Cal (Calpurnia), their cook, who has raised the children since the death of their mother when Scout was two. A collection is taken for the family of Robinson, the man Atticus is to defend. Aunt Alexandra, Atticus’s proper sister, comes to live with them to make a lady out of the tomboy Scout and restore proper southern order to their home.
Before the trial, the sheriff and a group of citizens warn Atticus that death threats were made against the defendant. Atticus stays at the jail and, weaponless, faces a mob come to get the prisoner. Jem, Scout, and Dill arrive, and Scout kicks a man who grabs Jem. She recognizes the father of a schoolmate in the mob and embarrasses him by talking calmly about his son, until the man orders the mob to leave. Atticus says the children made the schoolmate’s father stand in his shoes for a minute and turned the animals in the mob back into humans.
At the trial, where Scout, Jem, and Dill sit in the balcony with Calpurnia’s minister, Atticus demonstrates the untruth of the charges by Bob (Robert E. Lee) Ewell, a white man who lives on whiskey and welfare down by the dump, that Robinson beat and raped his daughter, Mayella. A doctor was not called to examine and treat the daughter, and the bruises on the right side of her face were caused by a left-handed man. Ewell is left-handed, and Robinson’s left arm is withered and useless.
Atticus asks Mayella on the witness stand if her father inflicted the abuse. She denies it, but Robinson testifies that the day of the alleged rape, she invited him in and kissed him. She said she never kissed a grown man—what her father did to her did not count—so she might as well kiss a “nigger.” Ewell arrived at that moment.
Jem and Scout believe that Robinson will be acquitted, but he is found guilty by the all-white jury. It is the word of a white person against a black one, and Robinson made the mistake of saying he felt sorry for a white person—Mayella.
After the trial, Ewell threatens Atticus in public. Robinson is killed after allegedly trying to escape from a prison exercise yard, giving up hope of getting justice in the white courts, although Atticus told him they had a chance on appeal.
Near Halloween, Scout and Jem attend a school pageant. On the way home in the dark, the children are attacked. Scout is saved from a knife thrust by the wire-mesh ham costume she is wearing. Jem struggles with the man and is thrown to the ground. A fourth person appears; there is a struggle, and Scout sees Jem being carried to their house by the stranger. Back home, Scout finds that Jem has a broken arm and the “stranger” who rescued him, standing silently in a corner, is Boo Radley.
The sheriff finds Ewell dead where the attack occurred, with a kitchen knife stuck up under his ribs. Atticus says that he believes Jem did it and does not want it covered up. The sheriff insists that Ewell fell on his own knife, and, besides, it would be a sin to drag someone with shy ways into the limelight. Atticus gives in and thanks Boo Radley for his children’s lives. Scout says it would be “sort of like shootin’ a mockingbird” to expose their rescuer.
Scout escorts Boo Radley home. She never sees him again. Atticus, putting her to bed, says that most people are nice “when you finally see them.”