Jean Louise "Scout" Finch, now 26, returns from New York to the fictional town of Maycomb, Alabama, on her annual two-week visit. She is met by her childhood sweetheart and suitor, Henry "Hank" Clinton, who works for her father, Atticus, a lawyer and former state legislator. Jack, her father's brother, a retired doctor, is her mentor. Their sister, Aunt Alexandra, runs the house since Calpurnia's retirement. The Supreme Court's Brown v. Board of Education decision and the National Association for the Advancement of Colored People (NAACP) are introduced as sources of controversy in the community.
Returning from a trip to Finch's Landing, Jean Louise and Henry are passed by a car of black men travelling dangerously at high speed. Henry mentions that the black people in the county now have money for cars but neglect to get licenses and insurance. The next day is spent dealing with the minor scandal a swim the previous evening has caused, and there are sequences during Jean Louise's youth, spent with Charles Baker "Dill" Harris, and her older brother Jeremy "Jem" Finch, who has since died of a heart condition which also killed her mother.
After finding a pamphlet titled "The Black Plague" among her father's papers, Jean Louise follows him to a Citizens' Council meeting where Atticus introduces a man who delivers a racist speech. Jean Louise watches in secret from the balcony and is horrified. She is unable to forgive him for betraying her and flees from the hall.
After having a dream about her old family black maid Calpurnia, whom she sees as a mother figure, Jean Louise has breakfast with her father. They soon learn that Calpurnia's grandson killed a drunk pedestrian the previous night while speeding in his car. Atticus agrees to take the case in order to stop the NAACP from getting involved. Jean Louise visits Calpurnia and is treated politely but coldly, causing her to leave, devastated.
At lunch with her Uncle Jack, Jean Louise questions why Atticus was at the meeting. Jack says that Atticus hasn't suddenly become racist but is trying to slow federal government intervention into state politics. Her uncle lectures her on the complexity of history, race, and politics in the South, trying to get Jean Louise to come to a conclusion, which she struggles to grasp. She has a flashback to when she was a teenager and recalls an incident where Atticus planted the seed for an idea in Henry's brain, then let him come to the right conclusion on his own.
While having coffee with Henry, Jean Louise tells him she doesn't love him and will never marry him. She expresses her disgust at seeing him and her father at the council meeting. Henry explains that sometimes people have to do things they don't want to do. She screams that she could never live with a hypocrite, only to notice that Atticus is standing behind them, smiling.
During a discussion with his daughter, Atticus argues that the blacks of the South are not ready for full civil rights, and the Supreme Court's decision was unconstitutional and irresponsible. Although Jean Louise agrees that the South is not ready to be fully integrated, she says the court was pushed into a corner by the NAACP and had to act. She is confused and devastated by her father's positions as they are contrary to everything he has ever taught her.
She returns to the family home furious and packs her things. As she is about to leave town, her uncle comes home. She angrily complains to him, and her uncle slaps her across the face. He tells her to think of all the things that have happened over the past two days and how she has processed them. When she says she can now stand them, he tells her it is bearable because she is her own person. He says that at one point she had fastened her conscience to her father's, assuming that her answers would always be his answers. Her uncle tells her that Atticus was letting her break her idols so that she could reduce him to the status of a human being.
Jean Louise returns to the office and makes a date with Henry for the evening. She reflects that Maycomb has taught him things she had never known and rendered her useless to him except as his oldest friend. She goes to apologize to Atticus, but he tells her how proud of her he is. He hoped that she would stand for what she thinks is right. She reflects that she didn't want her world disturbed but that she tried to crush the man who is trying to preserve it for her. She tells him that she thinks she loves him very much. As she follows him to the car, she silently welcomes him to the human race, seeing him as just a man for the first time.