A mysterious sea monster, theorized by some to be a giant narwhal, is sighted by ships of several nations; an ocean liner is also damaged by the creature. The United States government finally assembles an expedition to track down and destroy the menace. Professor Pierre Aronnax, a noted French marine biologist and narrator of the story, master harpoonist Ned Land, and Aronnax's faithful assistant Conseil join the expedition.
After much fruitless searching, the monster is found, and the ship charges into battle. During the fight, the ship's steering is damaged, and the three men are thrown overboard. They find themselves stranded on the "hide" of the creature, only to discover to their surprise that it is a large metal construct. They are quickly captured and brought inside the vessel, where they meet its enigmatic creator and commander, Captain Nemo.