Tom hears good news on Friday morning: The Thatcher family has returned from their family vacation. Tom spends most of the day with Becky, and she talks her mother into the promised picnic for the next day.
Invitations are sent out, and everyone gathers the next morning for the chartered ferryboat. Mrs. Thatcher decides that, because it will be late, Becky should stay with a friend near the ferry landing. Tom talks Becky into joining him for ice cream at the Widow Douglas' house after the ride, and Becky reluctantly agrees.
Three miles down river, the ferry boat stops, and everyone plays until someone shouts that it is time to try McDougal's cave, a "vast labyrinth of crooked aisles." Everyone knows some of the cave--and Tom knows as much as anyone did--but no one alive knows the entire cave. After much wandering about, the group finally leaves the cave to discover that it is almost dark, and the ferryboat is anxious to make the trip home.
That night, while Huck watches "Number Two," the door opens, and two men brush past him. Because he does not have enough time to give Tom Sawyer the signal, Huck carefully follows the men, who go to the quarry. When the men stop, Huck hides and listens. He overhears Injun Joe planning revenge: Some years earlier, Judge Douglas, Widow Douglas's late husband, had Injun Joe horsewhipped in public. To get even, Injun Joe plans "to slit [the widow's] nostrils and notch her ears like a sow."
The two men see a light in the Widow's house and, thinking that she has company, decide to wait until later in the night. Huck silently creeps away and runs frantically to the Welshman's house, which is close by. He tells what he has heard and makes the Welshman and his sons promise not to tell who told them. They all leave for the sumac bushes with Huck lingering behind. When he hears shots, he waits no longer and runs back to town as quickly as possible.
As soon as it is daylight, Huck goes back to the Welshman's house. The Welshman is glad to welcome Huck into his house because of Huck's courage and because he prevented the Widow from being mutilated. Huck hears how the Welshman and his sons hid behind a spot in the sumac bushes and were only fifteen feet away from Injun Joe when an unfortunate sneeze came upon the Welshman. The robbers ran away. The Welshman and his men fired after them and pursued Injun Joe and his partner, but the men escaped capture.
Huck explains to the Welshman how he had seen the two robbers and had followed them and overheard them talking about mutilating the Widow Douglas' face. Under pressure, he reveals the identity of Injun Joe, and the Welshman promises to protect him from this vicious man. A loud knock at the door causes Huck to jump almost out of his skin. It is the Widow Douglas and a group of citizens who want to express their gratitude to the Welshman. Mr. Jones (the Welshman), in turn, tells the widow that "There's another that you're more beholden to el but he don't allow me to tell his name."
At church that morning, Mrs. Thatcher discovers that Becky is missing. Shortly after that, Aunt Polly discovers that Tom is also missing. The people realize that they are still in the cave. Two hundred men are gathered, and they immediately go to the cave to begin the search. On Monday, the many men return, and Huck is found sick with a high fever. The Widow Douglas comes to care for him. When Huck hears a discussion about the Temperance Tavern break-in, he jumps up from his fever and asks if anything had been found. He is told that only whiskey had been found. When he asks about Tom Sawyer, they keep Tom's disappearance from him.
The search for Tom and Becky continues for three days. They find a hair-ribbon and the children' names lettered on the wall--proof that they are still in the cave. Despair settles in when the men no longer have either hope or energy to keep looking.
These two chapters describe the various activities of the entire group in the cave, and the return home, then abruptly change to a narration about Huck Finn's adventures in town.
Twain does not return to Tom and Becky's adventures in the cave until Chapter 31, but the reader must remember that both Tom and Becky are in the cave with its many passageways and unknown areas. Although the reader is not aware of it yet, this is the scene of Tom's most perilous adventure and the only one that involves Becky Thatcher. Earlier, Mrs. Thatcher had arranged for Becky to stay with the Harper family, and as a result, Becky and Tom are not missed until church the following morning.
With Tom in the cave, we see Huck Finn acting on his own. Until this point, we have only seen Huck in relation to Tom, and when Tom is around, Tom is the leader of the two. He is the one with the education; he is also the one who is a respected member of society and the one who is expected to know what should be done in any situation. Huck has always been agreeable to Tom's suggestions and leadership, and he does what Tom wants him to do. However, now that Huck in on his own, we see that he is naturally smart, shrewd, and resourceful.
Huck's following Injun Joe to the hideout is a dangerous and frightening task, but he follows the two men carefully and overhears their plan to mutilate Widow Douglas' face, tie her up, and in other ways harm her. Huck's integrity cannot allow this. However, because he is not welcomed in the homes of the townspeople, he goes to awaken the Welshman, Mr. Jones, and tell him what he has heard. Thus, we see Huck doing the "right thing" in protecting the Widow Douglas because he is both fond of her and does not wish to see her hurt. This leads to the Widow's desire to protect and take care of Huck.
Huck's reception and welcome by the Welshman, Mr. Jones, is the most kindness and attention he has ever received, and Huck is not insensitive to this kindness. After the Welshman and his sons have chased Injun Joe away and when Huck returns early the next morning to discover the result--that is, what could have happened to the treasure--the Welshman greets him by commenting that the name of Huck Finn can "open this door night or day el and welcome." Huck cannot remember ever hearing the word "welcome" by anyone before and is deeply pleased with such kind treatment. Likewise, when he becomes ill, the Widow Douglas comes to care for him because of her own innate kindness. The reader should remember that she does not know at this time that it was Huck who saved her life. Here and elsewhere, Twain begins to be more interested in the character of Huck Finn--his native intelligence and his good moral sense. In fact, Twain's treatment of Huck in Tom Sawyer foreshadows the later novel, Adventures of Huckleberry Finn.
Injun Joe's evil and wicked nature is again emphasized. He so strongly feels the need of revenge that he will take out his evil hatred on an innocent person.