A girl called Scout (Jean Louise) Finch lives in a small town in Alabama during the '50s. Her father, Atticus, is a lawyer, and she has an older brother named Jem. The two kids with their friend Dill become really interested in the neighborhood "spook," a man named "Boo", really Arthur, Radley who appears never to leave his house. The children half-believe the silly tales that Boo haunts the neighborhood at night, feeding on squirrels, etc. As the kids grow up, they occasionally find little gifts in the knothole of an oak tree near their house. Atticus Finch, who is a lawyer, agrees to defend Tom Robinson, an upstanding black citizen accused of raping Mayella Ewell. Everyone in Maycomb (the town) knows that the Ewells are trash, and in the course of a very controversial trial, it becomes clear that Mayella's father Bob Ewell really raped her. Nonetheless, the jury convicts Robinson and sends him to jail, where he is later shot trying to escape. Bob Ewell is angry with Atticus for having humiliated him. On Halloween, as Jem and Scout are returning late from a Halloween party (Scout dressed as a ham), Ewell attacks them in the woods and Jem breaks his arm and falls on Scout, immobilizing her. But just as Ewell is about to strike, Boo Radley stabs him. As Radley has a mental deficiency, Atticus and the police inspector determine that it would be pointlessly cruel to put Boo on trial (like killing a mockingbird). At the end, Scout realizes that Radley has been watching them for some time and has been leaving them the gifts in the knothole.
The book talks about prejudice, specifically prejudice against black people and people with mental disabilities, but it's message is decidedly not limited to these two parties.