In different parts of the ocean, a number of ships sight a mysterious monster, gleaming with light, such as no man ever saw. After this monster attacks and sinks several vessels, people all over the world are both amazed and alarmed. Finally an American frigate, the Abraham Lincoln, is fitted out to find and to destroy the mysterious sea creature. Among its passengers is Pierre Aronnax, professor of natural history in the Museum of Paris, who published his opinion that the monster is a giant narwhal. One of the crew is Ned Land, an expert harpooner. For quite a while, the ship sails without sighting anything even remotely resembling the reported terror of the seas.

The creature is sighted at last. When an opportunity presents itself, Land throws his harpoon, but the monster is uninjured, and Land realizes that it is protected by a thick steel-like armor. During a pursuit in the darkness, a terrific explosion rocks the ship. Aronnax, Land, and Conseil find themselves floundering in the water. Aronnax faints. Regaining consciousness, he discovers that they are aboard some sort of underwater craft. Later, two men come to greet them. The survivors from the ship speak to them in various languages, but the men appear not to understand. Then the captain of the vessel appears and speaks to them in French. He reveals that his name is Nemo, that the vessel is a submarine, and that they are, in effect, prisoners who will have every liberty aboard, except on occasions when they will receive orders to retire to their cabins.

Aronnax learns that the submarine Nautilus was built in a complicated manner. Parts of it were secured from various places and secretly assembled on a desert island. Then a fire was set to destroy all traces of the work done there. The ship manufactures its own electricity, has provisions for quantities of oxygen that allow it to remain submerged, and is as comfortable as any home. All food comes from the ocean. There is fish, but fish such as Aronnax never before tasted. There is clothing made from some sort of sea fibers. There are cigars, not of tobacco but of a special seaweed. Captain Nemo shows them air guns that allow him and the crew to go hunting as well as a device that permits the crew to walk the ocean floor.

In the Pacific, Captain Nemo invites the three survivors to a hunt in the marine forest of Crespo, where Land saves Nemo’s life by killing a creature that is about to put an end to the captain. Later, the captain saves Land’s life. In Ceylon, they watch the pearl divers in the oyster beds. There Nemo saves a native from the jaws of a shark.

Off the coast of Borneo, the three survivors decide to go ashore in the hope of bagging some land game. While they are hunting, they are attacked by natives. Although they manage to get back to the Nautilus, the natives remain clustered about the ship. Aronnax is alarmed, certain that the natives will board the submarine when the hatches are opened for oxygen the next morning. He takes his problem to Nemo, who is not at all worried. Instead he tells the professor about a similar experience. Once, when the hatches were opened, natives attempted to come aboard, but the few who touched the rails let out a shriek and retreated in terror. Land touches the rail and is paralyzed with shock; the rail is electrified.

The captain announces suddenly that he will enter the Mediterranean Sea. Aronnax supposes...