For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Aronnax and company go searching for a mysterious sea creature. They end up captives in the belly of the (mechanical, man-made) beast.
Act II
With the help of the enigmatic Captain Nemo, Aronnax explores strange underwater worlds. He's blown away by what he sees. Still, some troubling incidents lead Aronnax to doubt his host's intentions. Meanwhile, Ned gets homesick and begins to plan an escape.
Act III
Nemo's troubling behavior is brought into the open. He sinks a ship violently and without remorse. Aronnax is appalled, and decides to escape with Ned and Conseil. When the Nautilus gets caught in the Maelstrom—an epic whirlpool that threatens to devour them all—they make a break for it and end up safe in Scandinavia. Nemo's fate is uncertain.