The film is narrated by the adult Jean Louise “Scout” Finch. Young Scout and her pre-teen older brother Jem live in the fictional town of Maycomb, Alabama, during the early 1930s. Despite the family’s modest means, the children enjoy a happy childhood, cared for by their widowed father, Atticus Finch, and the family’s black housekeeper, Calpurnia. During 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 father Nathan. The children have never seen Boo, who rarely leaves the house. On different occasions, 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 to defend 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. 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.
Atticus is appointed to defend Tom Robinson, a black man accused of raping a white girl, Mayella Ewell. Atticus accepts the case, heightening tension in the town and causing Jem and Scout to experience schoolyard taunts. One evening before the trial, as Atticus sits in front of the local jail to safeguard Robinson, 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. Cunningham 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. Among Atticus’ chief defensive arguments is that Tom’s left arm is disabled, yet the supposed rapist would have had to mostly assault Mayella with his left hand before raping her. Atticus noted that Mayella’s father, Bob Ewell, is left-handed, implying that he beat Mayella because he caught her seducing a young black man (Robinson). Atticus also states that Mayella was never examined by a doctor after the supposed assault. 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.
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 black spectators in the balcony rise to show their respect and appreciation.
When Atticus arrives home, Sheriff Tate informs him that Tom was killed during his transfer to prison, apparently while attempting to escape. Atticus, accompanied by Jem, goes to the Robinson home to relay news of Tom’s death. Bob Ewell appears and spits in Atticus’ 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. Doc Reynolds arrives and treats an unconscious Jem’s broken arm.
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. His official report will state that Ewell died falling on his knife. He refuses to drag the painfully shy, introverted Boo into the spotlight for his heroism, insisting it would be a sin. Scout draws a startlingly precocious analogy, comparing the unwelcome public attention that would have been heaped on Boo with the killing of a mockingbird that does nothing but sing. Scout escorts Boo home, never to see him again.