Aronnax enters the novel in Chapter 2, as he's about to ditch New York for his home country of France. But he never makes it back because he's invited by the U.S. Secretary to the Navy on a voyage to hunt down the narwhale-beast (or whatever it is) that's tearing up the world's oceans.
He and his servant, Conseil, hurry aboard the Navy frigate the Abraham Lincoln. A bunch of people wave them off from New York harbor, as always happens when famous scientists leave for monster-hunting expeditions.
Aronnax meets Ned Land, a French-Canadian, harpoon-wielding wildman. Since Land has some claim to Frenchy-ness, Aronnax deems the dude good enough to talk to. The two end up debating the veracity of the many claims that have been made about the supposed giant narwhale they're after.
Everyone's itching for giant narwhale sightings, but there're none to be found. Aronnax and Conseil think they'll be mocked upon returning to France because they set off on this failed journey to find some evidently nonexistent monster. The ship's captain convinces everyone to hang in there for three more days, and, right before the deadline, the beast emerges from the depths.
The "narwhale" seems to be emitting electric light, and spouting water and steam like no known narwhale before it. The gang's pretty shocked by all of this. They chase the mysterious beast, but it outruns them.
Then, it takes an unexpected rest. People fire at it, but their shells bounce off its skin; so does Ned's harpoon. Finally, the beast retaliates, shooting water at the crew and knocking Aronnax into the water.
Conseil saves Aronnax, dragging him to the surface. They tread water; Aronnax freaks out because he thinks they're totally obviously going to die, but Conseil keeps his cool and looks after him.
Eventually they find Ned, who's standing on this "floating island," which turns out to be the steel-plated "beast" they've been battling. Suddenly, eight men grab the three and drag them down into the enigmatic vessel.
Aronnax and his buds are in some dark cell and have no idea what's happening. Two people, one of them lookin' like a captain, come down to look at the three men, but they don't seem to speak any recognizable languages. At least Aronnax and company have dinner brought to them in their cozy cell.
When Arronax awakes from sleeping, he feels ill. He thinks is because the oxygen levels in the sub are running low. He gets scared, but fresh air soon rushes into the cell. Ned wants to escape, but Aronnax and Conseil are freaked out that they're underwater in some newfangled technology. And besides, they're starving. Soon, the captain guy comes back, and shows that he can speak and understand French, Latin, etc. after all. He was just messing with the three newbies.
The Professor and his boys get told by Nemo that they can chill on his sub so long as they never, ever leave. And they let Nemo throw 'em back in their rooms and lock 'em up whenever he pleases. Seeing few other options at the moment, they agree to the arrangement.
Aronnax is amazed by all the food Nemo seems to collect under the sea, and even more amazed by what's aboard the Nautilus, from the library to its electric insides.
Nemo brags to Aronnax a ton about how awesome the Nautilus is, and how rich he is.
And now for the big reveal. Aronnax and company are utterly stunned by the Nautilus's giant viewing windows. He develops a new pastime: Fishie Observation with My Bros.
Invited by Nemo on a hunting trip in the forests of Crespo Island, Aronnax is confused. He thought the Captain Dude never went on land. Turns out, he doesn't; the forests are actually undersea.
Aronnax and his buddies don deep-sea diving suits (retro-style), and tromp towards the underwater forests. They almost get eaten by sharks, but don't. It's back to the sub for them.
Next, our protagonist gets exposed to some shipwrecks before climbing on land—gasp—when the sub gets stranded on some coral.
Aronnax and company do some fishing and hunting before getting run off by cannibals. But Nemo electrifies the little dudes and Aronnax and his friends go on their way.
Soon, the trio gets locked up on one of Nemo's whims. When he wakes up, Aronnax realizes they were drugged. Nemo asks Aronnax to help some sailor that was injured by an engine malfunction, but even Aronnax isn't dense enough to buy that excuse. The sailor dies and the bury him in a whack undersea grave.
Then Aronnax agrees, along with Conseil and Ned, to go pearl diving. Too bad the pearls, which Nemo seems to be feeding so they grow big and expensive for him, are guarded by hoards of sharks. An Indian diver who is also after the pearls gets attacked by a shark, but Nemo kills it. Then another shark attacks Nemo, but Ned kills it. Then, because he's all into aiding the underdog, Nemo gives the Indian a bunch of pearls.
Aronnax continues his love-hate relationship with Nemo (and his oceanic smarts) when Nemo reveals that he discovered this undersea passage between the Red Sea and the Mediterranean.
Aronnax keeps arguing with Ned about if/when they should escape. Because this whole world of sea-farin' adventures is just too cool for Aronnax to pass up, even if they are all being held captive by a crazy person.
He sees some underwater volcanoes, and watches as they pass through the Strait of Gibraltar.
Aronnax spies on Nemo, and digs around his room for some keys to unlocking the dude's mysteries. He finds a bunch of freedom fighters' pictures in Nemo's room, and gets to thinking that maybe he fought against the South in the American Civil War.
Then Nemo shows Aronnax some more shipwrecks that act as his private bank.
Then he explores some underwater, volcano-y caverns, and watches as Nemo kills a bunch of sperm whales (because they were trying to eat the Antarctic whales). We think it's pretty odd to kill one species of whale in favor of saving another species of whale; so does Aronnax. He's starting to catch on to Captain Nemo's blind attachment to "the oppressed."
Later, down at the South Pole, it's all ice, ice baby. First, the sub gets stuck in ice, and then underneath it. Aronnax works himself into a tizzy, what with being about to die from suffocation and all.
But the sub manages to resurface again, and Aronnax is back to his fish-observing ways.
Aronnax and company soon get into serious trouble again, though, when they are attacked by a giant squid. Good thing Ned's there to save the day.
After all these trials and tribulations and near-death experiences, Ned is so past ready to go home, so he bugs Aronnax to ask Nemo for their freedom. He does, Nemo says no, and, as a result, you can cut the tension between these two boys with a knife.
Aronnax finally agrees it's time to gtfo, but the Nautilus's movements and the weather keep thwarting them. Aronnax begins to wonder if Nemo is onto their escape plan and trying to thwart them.
In the big crescendo, an unidentified warship attacks the Nautilus and Nemo retaliates by killing everyone on board. He shows no mercy. Aronnax is, at long last, convinced that Nemo is a deeply disturbed dude, so Aronnax, Ned, and Conseil agree it's really time to go.
Aronnax runs past a freaky deaky Nemo, sitting in the dark and murmuring to himself, to get to the escape dinghy with Ned and Conseil. But at that moment, the sub is being pulled into the Maelstrom—a giant whirlpool of death and despair.
Aronnax is pretty sure Nemo couldn't have just gotten the Nautilus into such peril by accident; he's too good of a captain for that. But he's got bigger problems than Nemo's nuttiness at the moment—chiefly, how to not die. He's hit unconscious before he gets to do much in the way of saving himself.
When Arronax wakes up, he's in a fisherman's cabin off the coast of Norway. He waits around for a ferry so he can head back to France, and in the meanwhile, he decides to write a book about all of the adventures they've just had (a.k.a. the 20,000 Leagues we now know and love. Or hate. It's up to you, really.).
Aronnax spends some words wonderin' what in the world happened to Nemo, and hoping that if he survived, this whole Maelstrom experience will have made him change his ways. Nothing like a little unstoppable sea-sinkhole to make a man realize the errors of his vengeful ways, right?