On a quest for the "giant narwhal," Aronnax, Conseil, and Ned Land clash with a mysterious beast. They are thrown overboard the USS Abraham Lincoln. After some struggling, the three men end up on the so-called creature's deck; turns out, it's actually a submarine. The group is soon captured by the vessel's crew and thrown into a cell.
In this case, the fall is literal: our protagonists fall into the water, the domain of the Nautilus, and so being their journey. The Nautilus itself is not what Aronnax expected, but it's still a mysterious vessel—just the sort of thing he hoped to encounter on this journey.
After a strange initial encounter with the vessel's captain, Aronnax and company are finally let in on the secrets of the Nautilus. Nemo shows them fascinating things both within the Nautilus—its inner workings, the secret of its incredible power—and outside of it—the "forests" of Crespo, an undersea tunnel, Atlantis, the South Pole.
The "initial fascination" in 20,000 Leagues lasts a long, long time because there's so much stuff to be, well, fascinated by. The whole journey is like a lengthy dream; the sights they see are outrageous enough that, even after months of traveling, they're still surprised by what they see.
Really strange things start happening now. Nemo drugs Aronnax, Conseil, and Land, and throws them in a cell. A sailor dies and Nemo's clearly lying about how he ended up dead. Ned is super tired of being cooped up in the sub. Aronnax is very torn between his desire to explore the undersea world and his desire for freedom.
Though Aronnax is somewhat troubled from the very beginning of their voyage, his doubts multiply as the voyage goes on. The real problem is that his doubts don't dampen his curiosity. He's caught between his scientific tendencies and the survival instincts of any normal, land-dwelling being.
Aronnax troubles get very real, very fast. A scary battle with a giant squid is just an appetizer for a really disturbing main course: Nemo's vengeful destruction of a warship. From there on in, Nemo wanders the oceans, depressed and angry.
This is where the scales tip in favor of Ned's escape plan. Aronnax has a hard time focusing on his scientific pursuits when people are dying. He begins to worry about Captain Nemo's sanity and his own safety.
Captain Nemo sails right into the middle of the Maelstrom—perhaps by accident, perhaps not. Aronnax, Conseil, and Ned make a miraculous escape and end up in Norway. At the story's end, Aronnax is finishing his book and waiting to head back to France. As for Nemo, his fate's a mystery.
Quite a thrilling escape, eh? When we leave our main characters, they're simply waiting to get back to mainland Europe. For all intents and purposes, they're safe and sound. They've survived this wild n' crazy ordeal.