Narration is by the adult Jean Louise "Scout" Finch. Young Scout and her older brother Jem live in Maycomb, Alabama, during the early 1930s. Despite the family's modest means, the children enjoy a happy childhood with their widowed father, Atticus Finch, and their African-American housekeeper, Calpurnia.
Over the summer, Jem, Scout, and their friend Dill play games and often search for Arthur "Boo" Radley, an odd, reclusive neighbor who lives with his brother Nathan. The children have never seen Boo, who rarely leaves the house. Occasionally, Jem has found small objects left inside a tree knothole on the Radley property. These include a broken pocket watch, an old spelling bee medal, a pocket knife, and two carved soap dolls resembling Jem and Scout.
Atticus, a lawyer, strongly believes "all people deserve fair treatment, in turning the other cheek, and in defending what you believe." Many of Atticus' clients are poor farmers who pay for his legal services in trade, often leaving him fresh produce, firewood, and so on.[5] Atticus' work as a lawyer often exposes Scout and Jem to the town's racism, aggravated by poverty. As a result, the children mature more quickly.
The local judge asks Atticus to defend Tom Robinson, an African-American accused of raping a white girl, Mayella Ewell and Atticus agrees. This heightens tension in the town and causes Jem and Scout to experience schoolyard taunts.
One evening before the trial, Atticus sits out front to safeguard Robinson, where a lynch mob arrives. Scout, Jem, and Dill unexpectedly interrupt the confrontation. Scout, unaware of the mob's purpose, recognizes Mr. Cunningham and asks him to say hello to his son Walter, her classmate. He becomes embarrassed, and the mob disperses.
At the trial, it is alleged that Tom entered the Ewell property at Mayella's request to chop up a chifforobe and that Mayella showed signs of having been beaten around that time. One of Atticus' defensive arguments is that Tom's left arm is disabled due to a farming accident years ago, yet the supposed rapist would have had to mostly assault Mayella with his left hand before raping her.
Atticus shows that Mayella's father, Bob Ewell, is left-handed, implying that he beat Mayella because he caught her seducing the young African-American defendant. He also states that Mayella was never examined by a doctor after the alleged rape. Taking the stand, Tom denies he attacked Mayella, but states that she kissed him against his will. He testifies that he had previously assisted Mayella with various chores at her request because he "felt sorry for her" – words that incite a swift, negative reaction from the prosecutor, and a gasp from the white audience.
In his closing argument, Atticus asks the all-white male jury to cast aside their prejudices and focus on Tom's obvious innocence. However, Tom is found guilty. As Atticus exits the courtroom, the African-American spectators in the balcony rise to show their respect and appreciation.
When Atticus arrives home, Sheriff Tate tells him Tom was killed during his transfer to prison, supposedly while attempting to escape. Atticus goes to the Robinsons’ to inform them of Tom's death, accompanied by Jem. Bob Ewell appears and spits in his face.
Autumn arrives, and Scout and Jem attend an evening school pageant in which Scout portrays a ham. After the pageant, Scout is unable to find her dress and shoes, forcing her to walk home with Jem while wearing the large, hard-shelled costume. While cutting through the woods, Scout and Jem are attacked. Scout's cumbersome costume protects her but restricts her vision. The attacker knocks Jem unconscious, but is himself attacked (and killed) by a second man, unseen by Scout. Scout escapes her costume and sees the second man carrying Jem towards their house. Scout follows them and runs into the arms of a frantic Atticus. Still unconscious, Jem has his broken arm treated by Doc Reynolds.
Scout tells Sheriff Tate and her father what happened, then notices a strange man behind Jem's bedroom door. Atticus introduces Scout to Arthur Radley, whom she knows as Boo. It was Boo who rescued Jem and Scout, overpowering Bob Ewell and carrying Jem home. The sheriff reports that Ewell, apparently seeking revenge for Atticus humiliating him in court, is dead at the scene of the attack. Atticus mistakenly assumes Jem killed Ewell in self-defense, but Sheriff Tate realizes the truth – Boo killed Ewell defending the children. However, he insists on declaring Ewell simply fell on his knife, refusing to drag the painfully shy and introverted Boo into the spotlight for his heroism. To Atticus' surprise, Scout agrees, pointing out that the unwelcome attention would be like killing a mockingbird that does nothing but sing.
