Jem, Scout, and their friend Dill are fascinated with their neighbor Arthur "Boo" Radley, a mysterious loner who never ventures outside of his house. The kids have heard rumors that Boo Radley eats raw squirrels and kills the neighbors' pets; although they have never seen him in person, they believe he has a large scar across his face and drools constantly.

The children paint him as a mythical menace; the reader, however, is informed that Boo's hermetic existence is a result of abuse from his father. As the novel progresses, the kids begin to see signs that their neighbor is not the demon they had imagined. While playing on the Radley property, Jem and Scout discover gifts that have been left for them in the nook of a tree. On a different occasion, Boo's brother Nathan chases the kids off of the property; in his escape, Jem tears and then loses his pants. He returns to find them sewn back together and hung neatly over the fence.

The children finally encounter Boo at the end of the novel, when he emerges from his house to rescue them from Bob Ewell's attack. Scout is shocked at his actual appearance, compared to the menace she had head about; he is frail, ghostly, and completely non-threatening.

After the town's revulsive display of racism at Tom's trial, Jem muses,

I think I'm beginning to understand why Boo Radley's stayed shut up in the house all this time...it's because he wants to stay inside.

The kids come to understand that Boo has been a victim of cruelty, that the outside world has treated him with the same callousness they showed Tom; they learn that there isn't anything inherently "wrong" with Boo and that the wrong lies instead with society at large.