The day after Tom and Huck witness Dr. Robinson’s murder, some townspeople discover the doctor’s corpse in the graveyard, along with Potter’s knife. A crowd gathers in the cemetery, and then Potter himself appears. To Tom, Huck, and especially Potter’s shock, Injun Joe describes how Potter committed the crime. Consequently, the sheriff arrests Potter for murder.

Tom’s pangs of conscience over not telling the truth about the murder keep him up at night, but Aunt Polly assumes just hearing about the horrid crime has upset him. Tom begins sneaking to the window of Potter’s jail cell every few days to bring him small gifts.

Becky Thatcher falls ill and stops coming to school. Tom’s depression worsens, so much so that Aunt Polly begins to worry about his health. She gives him various ineffective “treatments,” which culminate in an awful-tasting serum called “Pain-killer.” Tom finds this last treatment so intolerable that he feeds it to the cat, which reacts with extreme hyperactivity. Aunt Polly discovers what Tom has done, but she begins to realize that “what was cruelty to a cat might be cruelty to a boy, too,” and sends him off to school without punishment. Becky finally returns to school that morning, but she spurns Tom completely.

Feeling mistreated, Tom resolves to act on his earlier impulse to become a pirate. He meets Joe Harper, who is likewise disaffected because his mother has wrongly accused and punished him for stealing cream. They find Huck Finn, always up for a new adventure, and the three agree to slip away to Jackson’s Island, an uninhabited, forested isle three miles downriver from St. Petersburg.

That night, the three boys take a raft and pole their way to the island, calling out meaningless nautical commands to one another as they go. At about two in the morning they arrive on the island, build a fire, and eat some bacon that Joe has stolen for them. For the rest of the night they sit around and discuss pirate conduct. Eventually, however, they think about the meat they stole and reflect on the shamefulness of their petty crime—after all, the Bible explicitly forbids stealing. They decide that “their piracies should not again be sullied with the crime of stealing” and fall asleep.

Twain discourages us from feeling sympathy for Injun Joe, the novel’s most pronounced villain. We learn that Dr. Robinson once mistreated Injun Joe by chasing him off when he came begging one night, but Injun Joe’s willingness to murder a man as retribution for this relatively minor offense and his decision to pin the crime on a pathetic drunk who instinctively trusts him confound our ability to feel sorry for him.

Joe’s status as a “half-breed” (he is half “Injun,” or Native American, and half white) makes him an outsider in the St. Petersburg community. The novel contains racist suggestions linking Injun Joe’s villainy to the presumed contamination of his white blood. Joe tells Dr. Robinson, “The Injun blood ain’t in me for nothing,” suggesting that the alien, “Injun” part of Joe is what inspires his evil. When Injun Joe reappears in disguise later in the novel, he comes dressed as a deaf and mute Spaniard. In a way, Joe’s choice of disguise is logical, given his dark features, but the outfit also reinforces Injun Joe’s foreignness.

As in Chapter 8, Becky’s rejection turns Tom to thoughts of piracy. Twain mocks the convention in adult romances that unrequited love drives men to desperate acts. Only Huck, who joins Joe Harper and Tom as they act on Tom’s pirate fantasy, adds an authentic outlaw element to the adventure. Huck smokes and is something of an outsider in St. Petersburg society. However, whereas Injun Joe is completely ostracized by the St. Petersburg community, Huck Finn is allowed some mobility within it, as Huck’s roles—as Tom’s companion and, later, as the Widow Douglas’s adoptee—show.

The boys’ trip to the island and their plans for a pirate career demonstrate their imaginative energy and their innocence. Through several exchanges, the three reveal that they know very little about what being a pirate actually entails. The children’s books they have read furnish their entire conception of an outlaw’s life. Tom’s remarks about pirates that “they have just a bully time … [they] take ships, and burn them, and get the money and bury it in awful places [but] they don’t kill the women—they’re too noble” demonstrate the degree to which Tom idealizes these figures. Furthermore, the boys’ remorse over the stolen bacon—an actual, and comparatively small, offense—shows that they don’t see the storybook misdeeds they venerate as actual sins or punishable offenses. In their shame at having stolen the bacon, they defer to the Ten Commandments and to their own consciences, irrationally deciding that such mean behavior is unworthy of their idealized image of a pirate. Up to this moment, we have seen Tom maturing mentally, as he dreams up scheme after scheme. He has matured through his eye-opening experiences, such as his witness of Dr. Robinson’s murder, and he has matured emotionally, as he falls for and is rejected by Becky Thatcher. Tom’s rejection of sinful behavior, however, marks the first instance of his moral maturation. We know he has the capacity to memorize and imagine a whole new world of pirates on the high seas, but now we see that he understands right versus wrong as well.