Summer finally comes, but Scout is crushed when Dill doesn't arrive because his mother got remarried. To makes matters worse, Atticus has to leave for two weeks to serve in the state legislature.
By keeping Dill, a symbol of childhood innocence, away from Maycomb, Lee signals that innocence is ending.
Calpurnia, who's in charge when Atticus is away, invites Scout and Jem to attend her church that Sunday. The all-black congregation gladly welcomes the Finch kids, except for one woman Lula, who gets angry that Calpurnia brought white kids to their church.
Lee presents Lula's anger at Calpurnia and the children as wholly wrong—as just another form of prejudice. This stance that "prejudice is bad and wrong no matter what" is an attractive and simple one, but in the context of American society (particularly in the South) it can be overly simplistic. Given the treatment of blacks in Maycomb, Lula has good reason to be mistrustful of whites, and to be angry that white people have entered a place of black safety and solidarity. Since one side (the white side) has all the power, the argument that "everyone should just get along" also functions as an argument to maintain the status quo—that which keeps whites powerful and blacks powerless.
During the service, the congregation gathers money to support Helen, Tom Robinson's wife. Scout realizes Tom Robinson is the man Atticus is defending, and asks what he did. Calpurnia tells her: Tom has been accused by Bob Ewell of raping his daughter. Scout doesn't know what "rape" means, but can't believe anyone would trust the Ewells.
As a child and Atticus's daughter, Scout's view of the world hasn't been warped by racism. Her shock that anyone could trust the Ewells indicts the white people who trust Bob Ewell over Tom simply because Ewell is white.