There are two primary themes in Harper Lee's novel, To Kill a Mockingbird, and they both eventually intertwine in the dramatic climax. One theme concerns the mysterious Boo Radley and how he is treated by society. The other concerns the trial of Tom Robinson, a Negro who has been unfairly charged with raping the "white trash" Ewell, Mayella. Boo and Tom are the two adult "mockingbirds"--innocent and harmless--that are referred to in the title.
Boo's role as an outcast from society has always held an interest to Jem and Scout, and it grows greater upon the arrival of another castoff, Dill. They try mightily to lure Boo from his refuge in the Radley place, but they never get a look at him until the end. Atticus insists that the kids respect Boo's privacy, and the plot shifts to the Robinson trial during the second half of the novel.
Robinson is falsely charged with raping Mayella Ewell, who he had tried to help out of sympathy--a trait that no black man could afford to display in the 1930s South. Despite his disability and a stalwart defense by Atticus, Tom is found guilty. He is eventually shot by prison guards while trying to escape.
The two plots crisscross at the end when Boo is forced outside his home in order to save Jem and Scout from Bob Ewell, who plans to do harm to the Finch children in retribution for Atticus' verbal attack of his daughter on the witness stand. Boo kills Bob--or does he?--and returns to his home to live out his life in solitude. The head of the Ewell household is dead, but there is a whole new generation of his children to replace him.
So, Jem and Scout are saved; Boo becomes an invisible hero; and Bob Ewell will no longer harrass the residents of Maycomb. But Boo's gesture does not change his way of living, and there are more Ewells to remember their father's end. But for now, Sheriff Tate's reasoning will do.
"Let the dead bury the dead this time, Mr. Finch. Let the dead bury the dead."