Scout is ready to fight Cecil Jacobs on the schoolyard when he says that her father defends "niggers" (9.3).
(This is the word the book uses, so we'll use it here, despite its history of offensiveness. See the "Speech and Dialogue" section in "Tools of Characterization" for a fuller explanation of how this term functions in the book.)
When Scout asks Atticus about it, he tells her not to say "nigger."
Scout then asks him if all lawyers defend Negroes, and he says that of course they do.
So why does Cecil make it sound worse than bootlegging (booze, not music)? Atticus tries to explain to Scout the complexities of race relations in Maycomb.
See, just because lawyers have black clients doesn't mean they actually do a good job at defending them. But Atticus does.
For him, it boils down to self-respect: he couldn't hold his head up if he did less than his best.
Is he going to win the case? No, but they have to try anyway.
Atticus reassures Scout: "But remember this, no matter how bitter things get, they're [the residents of Maycomb are] still our friends and this is still our home" (9.27).
The next day at school, Scout is about to fight Cecil Jacobs when she remembers what Atticus told her and walks away instead, even though she gets called a coward.
Soon it's Christmas, which means a visit from Uncle Jack (good), but also a visit from Aunt Alexandra (bad).
Even worse, it means having to spend time with Aunt Alexandra's grandson Francis, who is the yin to Scout's yang.
Uncle Jack arrives with two long packages of mysterious contents.
Scout cusses while Uncle Jack's around, and later he tells her that she shouldn't do that if she wants to grow up to be a lady (which she doesn't).
The next day is Christmas morning, and they open the mysterious packages to find a pair of long-desired air rifles. (You'll shoot your eye out!)
They head down to Finch's Landing, sans air rifles (to Scout's dismay, as she'd already had fantasies about shooting Francis).
Jem abandons his sister to schmooze with the adults, leaving Scout to deal with the dreaded Francis—whose main problem so far seems to be liking boring Christmas presents.
Apparently Aunt Alexandra has strong ideas as to what girls should be and wear (frilly dresses) that are very different from Scout's (overalls).
Oh, here's the problem: eventually, Francis quotes Aunt Alexandra, calling Atticus a "nigger-lover" who's "ruinin' the family" (9.98).
Scout whales on Francis, gets in trouble with Uncle Jack, and then heads back home to sulk. Eventually, Uncle Jack asks Scout to explain her side of the story. When she explains, Uncle Jack wants to go beat up the little punk himself, but instead he just bandages her still-bleeding hand.
Later Scout overhears Uncle Jack and Atticus talking. Atticus tells Uncle Jack some things about children: answer them truthfully, and bad language is less dangerous than hotheadedness.
Atticus says that Scout needs to learn to control her temper because things are only going to get harder.
How bad are things are going to get? Really bad.
He also says that he'd rather not have taken the case, but once it was offered to him he couldn't refuse it in good conscience.
Atticus hopes he can get his kids through the case without their "catching Maycomb's usual disease"—going "stark raving mad when anything involving a Negro comes up" (9.187)—and that they will come to him if they have questions.
Atticus then tells Scout, still lurking around the corner eavesdropping, to go to bed. Years later, an older Scout realizes that her father meant her to overhear the conversation.