Tom is delighted on Friday morning to learn that Becky has returned to town. That same day they get together with their friends and play, and Becky convinces her mother to let her have a picnic the following day. A ferry is chartered for the occasion, chaperoned by older girls. And it is arranged that Becky will spend the night at Susy Harper's should they return from the picnic late.
Tom and Becky reunite and act like a more mature couple, leaving their petty squabbles behind them. To further prove their independence, they plan the picnic as an event with no adult chaperones.
On the way to the ferry Tom convinces Becky that after the picnic they should go to the widow Douglas's instead of the Harpers' because the widow will have ice cream. Becky is reluctant to disobey her mother, but eventually agrees to Tom's plan. Tom, meanwhile, feels a bit guilty that he won't be at home to listen for Huck's call, but tells himself that nothing will happen at the tavern that night. He doesn't tell Huck about the picnic.
Tom does not prove as capable of adult thoughtfulness as his fidelity to Becky suggests: he convinces Becky to lie and also abandons Huck in the job they planned together. He is regressing back to his selfish ways, and his treatment of Huck echoes how he ignored Amy Lawrence when Becky stole his attention.
The ferry carries the children three miles south of St. Petersburg, where they go ashore to play. After lunch, they decide to explore MacDougal's Cave, which is full of tunnels, and do so together as a group. Then the ferry returns them to St. Petersburg.
MacDougal's Cave's mysterious tunnels are straight out of one of Tom's fantasies. The children are either brave or foolish in venturing into it, though their decision to stick together as a group is wise and reflects Twain's theme of safety through community.
Watching from the tavern, Huck sees the ferry arrive, but is unaware of who it carries. He wonders whether the long hours spying are worth it, but just then two men exit the tavern carrying the treasure box. Huck makes a quick decision that there's no point in alerting Tom about what's happened if the treasure box isn't in the room. He decides to follow the men himself.
When Huck makes the quick decision to follow the two men himself he gives up his reliance on Tom to tell him what to do and takes on a new level of responsibility.
As Huck sneaks behind the men onto the Widow Douglas's land, the men suddenly stop, and Huck sees that they are indeed Injun Joe and the stranger. Injun Joe expresses his frustration at seeing lights on in the widow's house, as the lights suggest that she is hosting company. He speaks bitterly of how her deceased husband publicly "horsewhipped" him for vagrancy, and says he'll get revenge by mutilating the widow's face and tying her to her bed to bleed to death.
Injun Joe's desired revenge turns out to be directed at the widow Douglas, not Tom and Huck. His grudge is reprehensible, particularly because the widow Douglas presumably had no influence on her husband's actions. Throughout the novel, forgiveness has been held up as a great virtue. It is one that Injun Joe lacks.
Horrified, especially because the widow has been one of the few people in town to be kind to him, Huck runs to the Welshman's house. The Welshman is reluctant to let him in, given Huck's reputation, but eventually does. Huck tells what he has learned, asking that the Welshman never identify him as the source of the information. The Welshman and his two sons arm themselves and head to the widow's. Huck hears gunshots and a cry. He runs back to St. Petersburg.
Huck proves himself a hero, surpassing Tom as the bravest, or most mature, boy in the village. He braved imminent danger in tracking Injun Joe, and saved another villager's life despite the constant prejudice he has felt from the villagers. He also surpasses Tom in heroism for not expecting, or even wanting, credit and admiration for his good deed.