In the second half of this novel, Captain Nemo, along with the Nautilus and its hostages, encounter a series of ever so growing perilous encounters. After leaving the Mediterranean, the submarine continues through the Indian Ocean, where Professor Aronnax, Conseil, and Ned Land witness incredible nautical marvels. However, Ned begins to grow impatient of living in the Nautilus. Wanting freedom from his hellish aquatic prison, he plans to escape.

In the first half of 20,000 Leagues Under The Sea, the story is told from the first-person point of view of Professor Pierre Aronnax, a French naturalist. The story begins with tales of a mysterious sea creature wreaking havoc to ships at sea, leading to rumours of it being a sea monster. Professor Aronnax, along with his servant Conseil and Canadian harpooner Ned Land, is invited onto the USS Abraham Lincoln to hunt down the creature.