And here is how the story goes.
As the story begins, ships of several nations spot a mysterious sea monster, which some suggest to be a giant narwhal; the creature also damages an ocean liner. The United States government finally assembles an expedition in New York City to track down and destroy the menace. Professor Pierre Aronnax, an expert French marine biologist and narrator of the story, who happens to be in New York at the time, receives a last-minute invitation to join the expedition, and he accepts. Canadian master harpoonist Ned Land and Aronnax's faithful assistant Conseil are also brought aboard.
The rest of the story follows the adventures of the protagonists aboard the creature--the submarine, the Nautilus--which was built in secrecy and now roams the seas free from any land-based government. Captain Nemo's motivation is implied to be both a scientific thirst for knowledge and a desire for revenge on (and self-imposed exile from) civilization. Nemo explains that his submarine is electrically powered and can perform advanced marine biology research; he also tells his new passengers that although he appreciates conversing with such an expert as Aronnax, maintaining the secrecy of his existence requires never letting them leave. Aronnax is enthralled by the undersea vistas, but Land constantly plans escape.
They visit many places in the world's oceans, some known to Jules Verne from real travelers' descriptions and speculation, while others completely fictional. Thus, the travelers witness the real corals of the Red Sea, the wrecks of the battle of Vigo Bay, the Antarctic ice shelves, and the fictional submerged land of Atlantis. The travelers also don diving suits to hunt sharks and other marine life with specially designed guns and have a funeral for a crew member who dies when an accident occurs inside the Nautilus. When the Nautilus returns to the Atlantic Ocean, a "poulpe" (usually translated as a giant squid, although the French "poulpe" means "octopus") attacks the vessel and devours a crew member.
Throughout the story Captain Nemo is suggested to have exiled himself from the world after an encounter with his oppressive country somehow affected his family. Near the end of the book, the Nautilus is tracked and attacked by a mysterious ship from that nation. Nemo ignores Aronnax's pleas for amnesty for the boat and retaliates. He attacks the ship under the waterline, sending it to the bottom of the ocean with all crew aboard as Aronnax watches from the saloon. Nemo bows before the pictures of his wife and children and is plunged into deep depression after this encounter, and "voluntarily or involuntarily" allows the submarine to wander into an encounter with the Moskenstraumen, more commonly known as the "Maelstrom", a whirlpool off the coast of Norway. The three prisoners successfully seize this opportunity to escape, but the fate of Captain Nemo and his crew is unknown.