To Kill A Mockingbird by Harper Lee reveals how justice, for the people of Maycomb County, is not about fairness but about serving what this community sees as its specific needs, regardless of the rights of Tom Robinson. Harper Lee exposes the justice system and its dependence on the community in delivering a fair verdict; something that is impossible in Maycomb County because of deep-set discrimination, stereotypes and mistrust. Racial prejudice dominates the actions of the community and Tom Robinson's guilt is a foregone conclusion in the minds of the community despite the fact that they know that the Ewell family and especially Mayella's father, has questionable integrity.
Atticus knows from the beginning that he has no likelihood of successfully defending Tom. Tom's innocence is almost immaterial but as Judge Taylor appoints Atticus to defend Tom, there can be no discussion. Atticus mentions that, "John Taylor pointed at me and said, 'You're it.'" It is between chapter 8 and chapter 9 when this apparently takes place and in chapter 9 the reader becomes aware of Atticus's determination to do whatever he can. However, even the fact that Tom admits to feeling "pity" for Mayella Ewell will ultimately prove to be too much for the all-white jury to contend with and Tom will be found guilty.
Unlike the film version of To Kill a Mockingbird, which depicts Judge Taylor coming to Atticus's house to personally ask him to defend Tom Robinson, the Harper Lee novel has no specific meeting. The first mention of Atticus's acceptance comes during a conversation with his brother, Jack. He informs his brother that an acquittal is an impossibility, but
"Before I'm through, I intend to jar the jury a bit--I think we'll have a reasonable chance on appeal, though... I'd hoped to get through life without a case of this kind, but John Taylor pointed at me and said, 'You're it.' "
(This is found in near the end of Chapter 9.)
Oddly, this conversation between the two brothers is not overheard by Scout, so it is not part of her normal narrative. She does not learn that Atticus has been handed the case (rather than volunteering for it) until just before the trial.
"Lemme tell you somethin' now, Billy," a third said, "you know the court appointed him to defend this nigger."
... This was news, news that put a different light on things...
(This is found midway though Chapter 16.)
The trial of Tom Robinson hangs like an enormous weight over the Finch household in the chapters leading up to the actual court proceedings. Atticus, it is made clear in Chapter 9, is representing Tom, who has been unjustly accused of raping a white woman by the town's most virulently racist example of "white trash," Bob Ewell. Scout is troubled by accusations she has heard from others about her father's role in defending Tom, Atticus being an attorney and respected citizen of Maycomb. How and why Atticus came to be in this position, however, is only incrementally revealed. It is in Chapter 9, that Scout challenges her father for the reason he has taken such a highly-divisive case:
“If you shouldn’t be defendin‘ him, then why are you doin’ it?”
“For a number of reasons,” said Atticus. “The main one is, if I didn’t I couldn’t hold up my head in town, I couldn’t represent this county in the legislature, I couldn’t even tell you or Jem not to do something again.”
This sentiment -- and it is repeated in Chapter 11 when Atticus states, "This case, Tom Robinson’s case, is something that goes to the essence of a man’s conscience—Scout, I couldn’t go to church and worship God if I didn’t try to help that man" -- reveals the depth of Atticus' conscience and commitment to do what he believes is the right thing irrespective of its popularity and the ridicule to which it will inevitably expose his family. It is later in the series of exchanges Atticus has with his family, including Atticus' brother Jack, in Chapter 9, that Atticus refers to his appointment to this case by Judge John Taylor:
“Before I’m through, I intend to jar the jury a bit—I think we’ll have a reasonable chance on appeal, though. I really can’t tell at this stage, Jack. You know, I’d hoped to get through life without a case of this kind, but John Taylor pointed at me and said, ‘You’re It.’”
It is in Chapter 9, therefore, that Atticus reveals that he has taken this unpopular case because he was appointed to it by the presiding judge. Atticus could have, conceivably, turned down the judge's request that he defend Tom Robinson, crippled, desperately poor African American. It is emphasized, however, that he accepted the case because of a moral imperative to display for his children and to others his commitment to do what he believes is right.