How does the setting of Harper Lee's To Kill a Mockingbird affect the overall plot?

The setting of Harper Lee's To Kill a Mockingbird is central to the plot's development in this classic piece of literature.

Lee prepares the reader by describing not only the longstanding traditions of the imaginary town and surrounding area of Finch's Landing, but also by referring to the Civil War and inferring the virtually ineffectual hands of change seem to have passed by the town of Maycomb.

Scout describes Maycomb:

Maycomb was an old town, but it was a tired old town when I first knew it. In rainy weather the streets turned to red slop; grass grew on the sidewalks, the courthouse sagged in the square. Somehow, it was hotter then: a black dog suffered on a summer's day; bony mules hitched to Hoover carts flicked flies in the sweltering shade of the live oaks on the square. . . A day was twenty-four hours long but seemed longer. There was no hurry, for there was nowhere to go, nothing to buy and no money to buy it with, nothing to see outside the boundaries of Maycomb County.

The first image the reader receives is that of age. In this "tired old town," nothing seems to change. The "ambling" and "shuffling" of people gives the reader a sense that one day runs into the next, and no one seems interested in having things any different. There is no money and nowhere to go. The world stops at the boundaries of this county.

Harper Lee's allusion to FDR's pronouncement that we have "nothing to fear but fear itself" reminds the reader that this story occurs at the time of the Great Depression, when jobs are scarce and improvements of any kind—economic, social or political—are non-existent. In many ways, this setting provides the reader with the sense that progress is not to be desired and the status quo is perfectly fine with almost everyone.

There are many themes in the story: prejudice and tolerance, guilt and innocence, knowledge and ignorance, courage and cowardice, and loss of innocence.

The lack of change—or the lack of desire for it—permeates most of Maycomb. This area of the South still distinctly recalls its position with regard to the side on which they fought in the Civil War. While slavery is no longer allowed by law, racist attitudes towards blacks remain virtually unchanged. The poor live in one depressed area of Maycomb, but black people are segregated even from the poor whites. White and black people worship in different buildings. (Ironically, while the missionary society believes in helping the oppressed poor blacks in Africa, they have no tolerance for those that live in their own backyard.) Racial discrimination abounds in the town.

Prejudice and tolerance are themes central to the story's plot, seemingly more so than any other. While there are several subordinate plots evolving over the course of the novel, a social powder keg that Lee actively and zealously addresses deals racism and intolerance. We see it most strongly with the trial of Tom Robinson, who is accused of raping Mayella Ewell. Her father, Bob Ewell, is a raging bigot. The idea of his daughter making a pass at a black man is more demeaning than anything else he can imagine. Even though Tom has done none of the behaviors of which he has been accused, Ewell wants him dead. This attitude reflects the behaviors of the Deep South when blacks were enslaved and seen not as humans but as possessions; with this mindset, they had no rights and little value beyond the owner's monetary investment. While there are some members of the community who do not think this way (including Atticus, Heck Tate, Judge Taylor, and Maudie Atkinson), the majority of Maycomb's citizens are either racially-centered or lack the courage or conviction to stand up to those who are. 

Tom Robinson is found guilty even though there is no question of his innocence. Atticus does an excellent job of proving Tom could not have committed such a crime, but still the jury finds him guilty.

When Tom tries to run from the prison yard, he is shot 17 times.

“They shot him,” said Atticus. “He was running. It was during their exercise period. They said he just broke into a blind raving charge at the fence and started climbing over. Right in front of them—”

This response to Tom's attempt to flee is unconscionable, especially in light of Tom's physical handicap; his ability to escape is limited. Those who shoot him are not worried about Tom getting away as much as taking Tom out—killing him—with excessive force.

The plot is galvanized forward by racism and intolerance as Bob Ewell meets Atticus on the street after the trial has ended and spits in the lawyer's face. Unable to contain his violent hatred for Atticus because he defended a black man—while also exposing Ewell for the abusive father and lying reprobate he is—Bob Ewell even goes so far as to attempt to kill Jem and Scout (Atticus' children) as they walk home unescorted from a school function one evening.

Intolerance is also seen in the way the town treats Boo (Arthur) Radley. Forced to live in the seclusion of his home since he was a young man, people are intolerant rather than sympathetic. Gossips spread lies about Boo, even attempting to scare children with their wild tales about this innocent and abused man. Ignorance of the small town mentality is also easily linked to the novel's setting.

Mrs. Dubose is a bigot, accusing the children's father of reprehensible behavior in defending Tom Robinson.

Not only a Finch waiting on tables but one in the courthouse lawing for ni***rs. . . Your father's no better than the [black people] and trash he works for.

Atticus admires Mrs. Dubose's strength and commitment in light of her addiction to morphine because of her cancer. Jem cannot control his anger against Mrs. Dubose for her intolerance and racist remarks, however, especially those directed at his father, whom Jem idolizes.

Even Miss Maudie is the object of religious intolerance. The Baptist "foot washers" (as Miss Maudie calls them) are critical of Miss Maudie because she has flowers in her yard.

Footwashers believe that anything that's pleasure is a sin. . . Some of 'em came out of the woods one Saturday and passed by this place and told me me and my flowers were going to hell.

Prejudice and intolerance are at the center of nearly every element of the plot's development. It is as if Maycomb is frozen in time. This setting for the novel is developed to convey that with little change on the horizon and long-standing resentments still lingering with the end of slavery in the United States (almost 60 years after the fact), intolerance and racism are allowed to thrive. In numerous instances in the novel, this theme is easily identified.