The nightmare that descends upon Jem and Scout--and everyone else in Maycomb--is the trial of Tom Robinson. After spending "a week of peace together" with Dill after he had run away from home, bad things begin to happen on the weekend before the trial is scheduled to begin. Atticus receives word that Tom is about to be moved to the local jail and that there may be trouble from "that Old Sarum bunch." Sure enough, a lynch party appears at the jail just as the three children descend upon it to spy upon Atticus. The children know something is not right, but they don't realize that lives are in danger until the next morning. When the children sneak into the courtroom and observe the very adult facts of the rape trial, they become emotionally involved. Dill is brought to tears by Tom's treatment at the hands of the prosecutor, and Jem cries following the unjust guilty verdict. Bob threatens Atticus afterward, and even Tom's death does not end the children's nightmare, since Bob Ewell has plans for them on the following Halloween night. For Jem--and Bob--the nightmare ends tragically, though it is Boo Radley who appears as if in a dream to save the kids from Bob's murderous hands.
In Chapter 15 of To Kill a Mockingbird, the tone begins to change. It becomes more serious and Scout, the narrator, begins to see injustice in society. Scout refers to the time as a nightmare. This nightmare refers to the trial of Tom Robinson and the implications of it for Atticus and his family.
One night, the sheriff and a group of men from the area come to the Finch house. They ask to speak to Atticus. Sheriff Tate warns Atticus of possible violence because Tom is being moved to the jail in Maycomb. Atticus dismisses this idea:
"Don't be foolish, Heck," Atticus said. "This is Maycomb."
The next evening, Atticus sits outside of the jail where Tom is. An angry mob approaches. Scout, Jem, and Dill have snuck out of the house and are hiding near the jail. They run to Atticus when they see the mob. Scout speaks kindly to Mr. Cunningham, the leader of the group. Her kindness causes him to rethink his threats to harm Tom, and he orders the group to leave.
The trial then begins. Tom's trial is filled with bias because he is an African American man accused of a crime against a white woman. Scout and Jem witness the trial, which is like a nightmare to them as they see the injustice firsthand.