The Adventures of Tom Sawyer doesn't have a standard narrative structure. There are a number of different plots – you might call them "adventures" – some of them intersecting with and others entirely independent from the rest. What might make more sense is taking one of the plots from the book, let's say the cave sequence, and treating it as an independent storyline. Here goes.
Tom Sawyer and Becky Thatcher go on a picnic with their friends. After lunch, the boys and girls head into some nearby caves and go exploring.
Tom and Becky manage to get separated from the group. They get lost in an unexplored area of the sprawling caves. Soon they're tired and are close to running out of the candles, which they need to see.
Before the picnic, Becky told her mother that she was spending the night at a friend's house; Tom has told his aunt the same thing. As such, no one realizes that the two are missing until around noon the next day.
While looking for a way out of the cave, Tom sees someone holding a candle coming around the corner. He shouts for joy, only to realize that his would-be rescuer is in fact Injun Joe, a vicious murderer who already has a bone to pick with Tom. Luckily, Tom manages to avoid being seen and return to Becky.
Even after he escapes from Injun Joe, Tom still needs to get out of the cave; it's sort of an "out of the frying pan and into the fire…then right back into the frying pan" situation.
After some more searching, Tom finds a small hole at the end of a dark passage. He and Becky escape from the caves and make their way back to town. After recovering a little, Tom heads over to see Becky; when Becky's dad, Judge Thatcher, tells him that the cave's entrance has been sealed, Tom reveals that Injun Joe is in the cave.
The townspeople rush to the cave, where they find Injun Joe dead, his body sprawled on the ground behind the door; his attempts to dig his way out having failed.
At this point, the "Tom and Becky in the Cave" story can be said to have ended, but this paves the way for the ending of the "Tom and Huck Search for Treasure" storyline. Thus one relatively tight sequence allows another to progress. Tom goes into the caves to have some fun, but he comes out of them with the information he needs to find the treasure.