During the year 1866, ships of various nationalities sight a mysterious sea monster, which, it is later suggested, might be a gigantic narwhal. The U.S. government assembles an expedition in New York City to find and destroy the monster. Professor Pierre Aronnax, a French marine biologist and the story’s narrator, is in town at the time and receives a last-minute invitation to join the expedition; he accepts. Canadian whaler and master harpooner Ned Land and Aronnax’s faithful manservant Conseil are also among the participants.

The expedition leaves Brooklyn aboard the United States Navy frigate Abraham Lincoln, then travels south around Cape Horn into the Pacific Ocean. After a five-month search ending off Japan, the frigate locates and attacks the monster, which damages the ship’s rudder. Aronnax and Land are hurled into the sea, and Conseil jumps into the water after them. They survive by climbing onto the “monster”, which, they are startled to find, is a futuristic submarine. They wait on the deck of the vessel until morning, when they are captured, hauled inside, and introduced to the submarine’s mysterious constructor and commander, Captain Nemo.