The novel skips back in time to Saturday afternoon in order to tell the story of what happened to Tom and Becky. They're playing in the cave along with their friends. They split off to explore a more distant part of the cave together, and lose track of how far they've gone. They panic when they realize they can't find their way back because at some point Tom forgot to keep leaving smoke marks on the walls.
Tom thinks of himself as a family man and protector of Becky, but becomes so wrapped up in their imaginative play that he forgets the practical task of leaving smoke marks so that they can find their way back. He may be growing up, but he's not responsible yet.
Becky breaks down crying. Tom hugs and comforts her. His consoling works best when he blames the fiasco on his own incompetence. They wander again. Tom blows out Becky's candle in order to save it for later, and she admires him for his good sense. She naps a while, dreaming of a beautiful land. Waking, she assumes it might have been a vision of their future in heaven.
Tom does prove mature enough to accept responsibility for his actions. Yet in doing so, he and Becky continue with their fanciful play-acting, mimicking the male and female roles depicted in sentimental romantic novels that portray men as strong and powerful and women as weak and emotional.
They find a spring and drink some water. When Becky complains of hunger, Tom pulls out the only food they have, their "wedding cake" from the picnic. They dine in the light of their final candle. They realize that Becky's mother will not have noticed her missing, because she wasn't supposed to come home that night, which means there will be a delay in sending out search parties.
Tom and Becky had fantasized that the picnic marked the occasion of their wedding day. Their romantic fantasy is rudely interrupted by the realization that their parents will be significantly delayed in searching for them, which means they're in much greater danger than they had thought.
What must be days go by. Thinking he hears human noises, Tom leaves the spring, using the kite string he keeps in his pocket to track his route as he goes to look for others. When he sees a human hand resting on a rock, he cries out in joy. The hand turns out to be Injun Joe's, who flees at seeing Tom. Tom assumes Injun Joe didn't recognize him, for he would have killed him.
Becky is faint with fear and hunger. Tom doesn't tell her about seeing Injun Joe. Thinking a week may well have gone by, Tom continues to explore the passageways in hopes of being found. At this point, however, the starving pair expect to die.
Twain depicts Tom and Becky as acting tenderly and generously towards one another in the cave. Should they die, their deaths would indeed be tragic, for they have shown themselves to be good and loving souls.