Sneaking out at night to the graveyard, Tom and Huck Finn are horrified when they witness Injun Joe murder Dr. Robinson. Muff Potter is also there, and Injun Joe uses Potter's knife to commit the crime. Injun Joe tells Dr. Robinson he is getting revenge for Robinson telling his father not to feed him a meal five years ago.
Tom and Huck pledge an oath to each other not to reveal what they know of the murder because they fear Injun Joe might kill them if he finds out they are witnesses to the crime.
However, when Dr. Robinson's body is discovered, and Muff Potter is accused of the crime because his knife is the murder weapon, Tom's conscience is stricken. He knows it is wrong not to clear Potter by telling of what he saw. Yet he is afraid to speak. The situation leaves Tom upset. We learn that:
Tom’s fearful secret and gnawing conscience disturbed his sleep
Aunt Polly notices Tom is acting differently, and Sid says he hears Tom muttering about the murder in his sleep. Aunt Polly chalks it up to gossip about the murder scaring Tom the way it is everybody in the town. Sid, however, is watching Tom carefully and, from his behavior, knows that something else is up.