Go Set a Watchman is a coming of age story. Its theme or meaning is that as we grow up and become adults, we are confronted with the need to move from hero-worship of our parents to a realistic acceptance of their flaws. Maturity means loving people not because they are perfect but despite their imperfections. Scout struggles because she is faced with a deep imperfection in her father: does she reject her father because of it or accept him?
The adult Scout, in her mid-twenties, comes home from her job in the publishing industry in New York City to Maycomb, Alabama. She stays with her elderly father, Atticus, and her aunt Alexandria.
All her life, Scout has hero-worshipped Atticus as the perfect man, the man of honor. She thinks while she is at home that, unlike some of her friends, she has never had any contemptuous label for her father, such as calling him her "old man."
However, she comes across extremely racist literature in the house and finds out that Atticus is part of a racist organization that opposes the NAACP and racial integration. When she finally confronts him about it, she discovers he is a thoroughgoing racist who believes blacks are inferior and are not capable yet of sharing power with whites. She is utterly horrified, not only that her father is a racist, but that Atticus, a man she always admired as having the utmost integrity, the person who defended a black man unfairly accused of rape, could hold such views. A quote that describes her anguish is:
But a man who has lived by truth—and you have believed in what he has lived—he does not leave you merely wary when he fails you, he leaves you with nothing. I think that is why I’m nearly out of my mind.
Scout has to revise her view of her father from a paragon of perfection to a flawed man who she nevertheless can love.