Tom and Huck run back to the village, horrified by what they have seen. They stop and rest in an old tannery and discuss the implications. Huck says that if Dr Robinson is dead, someone will hang for it. If he or Tom tells anyone about the murder and Injun Joe escapes justice, he will certainly kill them. Huck suggests that they can let Muff Potter tell the authorities, but Tom points out that Potter was unconscious when Injun Joe killed Dr Robinson and cannot know the truth. Tom wonders if Potter is dead, but Huck believes that having a large amount of liquor inside a man makes him hard to kill.
Tom and Huck swear a blood oath not to tell about the murder. They write the oath on a piece of pine bark and sign it with their blood.
The boys hear a dog howling. At first they think the dog is howling at them, and are afraid because they think it means they will die. They soon realize that it is howling at a sleeping man, who turns out to be Muff Potter.
Tom goes home and creeps in at the window. Sid is awake and next morning he tells Aunt Polly about Tom's going out the previous night. When Tom goes downstairs for breakfast, Aunt Polly rebukes him for breaking her heart. Tom is filled with remorse, and promises to reform. He goes to school and he and Joe Harper are whipped for playing truant the day before. When he returns to his place, he finds the brass knob he gave to Becky left on his desk, wrapped in paper. He feels heartbroken.
The townspeople have heard about the murder of Dr Robinson, including the fact that Muff Potter's knife has been found near the body. Someone had come across Potter washing himself in the stream, and he had sneaked away, arousing suspicion. A search is launched for Potter. Tom follows the crowd to the graveyard. When the people see Dr Robinson's corpse, they all jump to the mistaken conclusion that Potter is the murderer. Then, Potter himself turns up and swears, before he is even asked, that he did not do it. This has the effect of making him sound guilty, since no one has publicly accused him yet.
Injun Joe tells his made-up story about Potter killing Dr Robinson to everyone present. Tom and Huck listen, dumbstruck, expecting the wrath of God to strike Joe down. Joe tells his story once again at the inquest.
For days afterward, Tom has problems sleeping because of his troubled conscience. Sid says Tom has been talking in his sleep about blood and threatening to "tell" about some unnamed secret. Aunt Polly concludes that Tom is simply upset at hearing about the murder. To appease his conscience, Tom takes to taking small gifts to Muff Potter in jail.
Becky Thatcher stops coming to school, and Tom hears that she is sick. Tom becomes miserable, and a concerned Aunt Polly tries out all the latest ludicrous medical fads on him, but in vain. Tom grows even more dejected. Aunt Polly decides that he needs a substance called Pain-killer, which was "simply fire in a liquid form." Tom secretly pours his doses of Pain-killer down a crack in the floor. One day, the cat shows an interest in the medicine, and Tom pours some into its mouth. The cat goes into a wild spin and tears around the house, howling. When Aunt Polly demands of Tom what he has done to the cat, he claims he dosed it out of pity, because he had no aunt to give him some.
Aunt Polly suddenly realizes that what constitutes cruelty to a cat may also be cruelty to a boy. She feels remorseful, and promises that he need not take any more medicine.
Tom goes to school and is delighted to see Becky Thatcher return. He shows off by hurtling around and yelling, falling literally at her feet, but she only turns aside in disdain. He feels crushed.
Analysis of Chapters 10-12
For the first time, Tom is suffering serious pangs of conscience for his actions - in this case, not telling anyone that Injun Joe, not Muff Potter, was the murderer, and thereby potentially allowing Potter to hang for a crime he did not commit. These pangs of conscience show that Tom is becoming more mature, in that he is thinking about the consequences of his actions or inaction.
Tom and Huck think up various ways of appeasing their bad consciences, in the hope that justice will somehow be done in spite of their own inaction. First, they take a blood oath binding them not to tell, on pain of dropping down dead. The seriousness of the oath leaves them with no choice but to keep silent, thereby somewhat removing the sense of personal responsibility. Later, when Tom and Huck hear Injun Joe telling everyone his made-up story about Potter murdering Dr Robinson, they expect "every moment that the clear sky would deliver God's lightnings upon his head." Tom and Huck are hoping that God will do the job they should have done. When it becomes clear that God is not prepared to act on their behalf, they conclude that "this miscreant had sold himself to Satan, and it would be fatal to meddle with the property of such a power as that." Again, this relieves them of the responsibility to act.
However, all of these ruses fail, and Tom suffers disturbed sleep as a result of his guilt. He avoids the 'inquests' that the other children play-act. Finally, he takes to sneaking small gifts to Muff Potter in jail in an attempt to ease his conscience, with some success. But the sheer difficulty he is experiencing in coming to terms with these issues shows that he is reaching an awareness of the difference between right and wrong.
In Chapter 12, in the incident in which Aunt Polly tries to rally Tom's spirits by trying out all the latest 'cures,' Twain satirizes medical fads and people's blind faith in them. He compares Aunt Polly to Death, one of the Four Horsemen of the Apocalypse in the Bible (Revelation Ch. 6). These horsemen are predicted to ride at the apocalypse, bringing death, war, famine and pestilence in their wake. The comparison is humorous because of its incongruity: the benevolent Aunt Polly is likened to the most terrifying of mythical figures. However, the comparison has a serious aspect to it, as countless people of Twain's time were made sick or killed by misguided medical practices and frequently toxic medicines.
Tom shows his psychological sophistication (as he did previously in the whitewashing episode) in giving Aunt Polly's Pain-killer remedy to the cat, which, naturally, goes wild with pain. The lesson reaches home: "Aunt Polly felt a pang of remorse. what was cruelty to a cat might be cruelty to a boy too."
When it comes to expressing his emotions to Becky, however, Tom is still woefully inept. When she returns to school after an absence, Tom can only try to attract her attention by showing off with handsprings and yelling. She is unimpressed, and Tom is aware that his approach has failed miserably.
The Adventures of Tom Sawyer: Novel Summary: Chapter 10 - 12