In the beginning of the story ships of several nations spot a mysterious sea monster; the creature also damages an ocean liner.

The United States government finally assembles an expedition in 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.

Canadien master harpoonist Ned Land and Aronnax's faithful assistant Conseil are also brought aboard... (Excerpt from Wikipedia)