Jean Louise "Scout" Finch, a single 26-year-old, returns from New York to her hometown, Maycomb, Alabama, for her annual fortnight-long visit to her father Atticus, a lawyer and former state legislator. Jack, her uncle and a retired doctor, is Scout's mentor. Alexandra, her aunt, moved in with her brother Atticus to help him around the house after Calpurnia, their housekeeper, retired. Jean Louise's brother, Jeremy "Jem" Finch, has died of the same heart condition which killed his mother. Upon her arrival in Maycomb, she is met by her childhood sweetheart Henry "Hank" Clinton, who works for Atticus. When returning from Finch's Landing, Jean Louise and Henry are passed by a car full of black men travelling at a dangerously high speed; Henry mentions that the black people in the county have money for cars but are without licenses and insurance.
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. Jean Louise finds a pamphlet titled "The Black Plague" among her father's papers. She 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 his behaviour and flees from the hall. After dreaming about Calpurnia, her family's black maid 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.
While at lunch, Jean Louise wants to know why Atticus was at the meeting. Uncle Jack tells her that Atticus has not suddenly become a racist but he is trying to slow down federal government intervention into state politics. Her uncle lectures her on the complexity of history, race, and politics in the South, in an attempt to get Jean Louise to come to a conclusion, which she struggles to grasp. She then 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. Jean Louise tells Henry that she does not love him and will never marry him. She expresses her disgust at seeing him with her father at the council meeting. Henry explains that sometimes people have to do things they don't want to do. Henry then defends his own case by saying that the reason that he is still part of the Citizens' Council is because he wants to use his intelligence to make an impact on his hometown of Maycomb and to make money to raise a family. 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 did not want her world disturbed but that she tried to crush the man who is trying to preserve it for her. Telling him that she loves him very much as she follows him to the car, she silently welcomes him to the human race. For the first time, she sees him as just a man.