After the pageant, Scout and Jem head home in the dark, but Jem thinks he hears someone following them. He pretends to think it's just Cecil and tells Scout to take off her ham costume so they can better get away. Before she can, though, a grown man runs at the from the darkness. Jem tells Scout to run, and she tries to, but falls in her costume. She hears fighting and Jem screaming, then silence.
When she goes back to investigate, she sees a man's body, smelling of booze (Bob Ewell). She also sees another man (Boo Radley), in the distance under a streetlight, carrying Jem, whose arm is broken.
Boo carries Jem to Atticus' house, and Scout follows. Atticus and Aunt Alexandra get busy calling the doctor and sheriff and seeing to Jem and Scout's injuries. When Mr. Tate shows up, he reports that the body of Bob Ewell was found stabbed to death, next to Scout's dress and fabric from her costume.
As Scout tells Atticus and Mr. Tate the story of the evening, they see that Bob Ewell was truly trying to hurt the kids; there is a slash in Scout's costume where he tried to cut her with a knife. Scout also realizes the identity of their rescuer, looking up at the shy, pale man in the corner and saying, "Hey, Boo." She acts as hostess to Boo, showing him around the house and making him comfortable, in what seems like a very surreal situation for her.
Atticus and Tate's conversation becomes legal here. Atticus thinks that Jem killed Ewell and begins talking about his court case. Tate suggests that Ewell probably fell on his own knife. Atticus can tell Tate is hiding something and it becomes clear, without anyone having said it aloud, that Boo killed Ewell. Tate won't put Boo through the publicity of a trial, though; as Scout says, it would be like killing a mockingbird.
When Scout walks Boo home, he returns to his house and she never sees him again.