Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Exposition (Initial Situation)
Aronnax Hunts a "Giant Narwhal"
When a mysterious creature begins wreaking havoc throughout the world's oceans, Dr. Pierre Aronnax, an expert on the undersea world, is asked to weigh in on the controversy. After telling the press that he believes a "giant narwhal" is responsible for the attacks, he is asked to go on a search mission put together by the US Navy. He accepts the offer and that leads to our…
Rising Action (Conflict, Complication)
Aronnax Is One Happy Prisoner
Aronnax, Conseil, and Ned's original ship gets wrecked, so they get onto the Nautilus and meet with Captain Nemo. He tells them they must never leave the Nautilus because he wants to protect its secrets, and his own. These three guys want their freedom, but at this point, they aren't really in any place to argue with Nemo. They're trapped underwater in a metal cylinder.
At least Aronnax and company are allowed to roam the ship. And Nemo acts as a sort of tour guide for Aronnax, showing him all the wonders of the undersea world. Aronnax is so amazed by these adventures that he begins to think he might never want to leave Nemo's sub.
As Nemo acts more and more erratically, Aronnax becomes increasingly torn between his desire for freedom and his desire to break new scientific ground. Ned and Aronnax fight a lot about escaping, because Ned would rather get the heck off of the sub as soon as possible. But the marvels of the ocean are so marvel-y that Aronnax has no desire to leave right away; he's okay with trading his freedom for the chance to roam Atlantis and the South Pole.
Climax (Crisis, Turning Point)
Nemo's Too Funky For… Everyone
When the Nautilus is attacked by an unidentified warship, Nemo has no trouble sinking it. Aronnax is horrified by Nemo's lack of guilt at killing everyone on board. Aronnax realizes that many of the most mysterious incidents that occurred during his journey—like when he, Ned, and Conseil were drugged and thrown back in the cell—can be chalked up to Nemo having some deadly encounters with his vague, unnamed enemies. Aronnax begins to see Nemo in a new light, and he decides he'd like to escape.
Suspense
Will They or Won't They?
After the attack, Nemo gets depressed, and the Nautilus wanders aimlessly through the Atlantic. Aronnax wonders if they'll ever get a chance to escape. At this point, Aronnax really starts to get antsy. He knows it's time to go, but he's afraid something bad may happen before he ever gets the chance.
Resolution (Dénouement)
Dangerous Waters
Just as Aronnax, Ned, and Conseil are about to escape, Nemo drives the Nautilus right into the Maelstrom. Aronnax's worst fears seem to be confirmed: it seems as though he's never going to have a chance to leave.
Our protagonists manage to survive, but Nemo's fate is unclear. The Nautilus trip into the Maelstrom is suspicious. As is true of lots of events in 20,000 Leagues, we never know whether this dramatic trip into the Swirling Whirpool of Doom is an accident or an incident. We are left wondering if Nemo is trying to write his own conclusion to the group's story.
Conclusion
Off the Sub
Aronnax, Ned, and Conseil wash up in Scandinavia. Aronnax puts the finishing touches on his manuscript—which is pretty much the book we've just read—and waits for a boat to take him back to the mainland. As for Nemo, well, who knows?
At book's end, Nemo remains as mysterious as ever. Aronnax is left with the same questions he had at the beginning. He hopes that the captain will change his vengeful ways, but he (and we) don't know if Nemo will get the chance.