The story begins when 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 assembles an expedition to track down and destroy the menace. Professor Pierre Aronnax is a noted French marine biologist and narrator of the story; as he happens to be in America at the time and is a recognized expert in his field, he is issued 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 along.

The expedition sets sail and after much fruitless searching, the monster is found. The ship charges into battle. During the fight, the ship's steering is damaged, and the three protagonists 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. It is here the adventure truly begins!