The title

The title refers to the distance traveled under the sea, not to the depth, since 20,000 leagues is more than ten times the radius of the earth. It is a common misconception that it refers to the depth. There is a passage in the book about this.

The Plot

The story was written before modern sea-going submarines were a reality. It is narrated by Professor Pierre Aronnax, a noted marine biologist, who is accompanied by his faithful assistant Conseil and a Canadian harpooner named Ned Land. As the story begins, 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. Since Aronnax happens to be in New York City at the time and is a recognized expert in his field, he is invited at the last minute to take part and he accepts. Master harpoonist, Ned Land and Aronnax's faithful assistant, Conseil were also brought on board.

The expedition sets sail from Long Island, New York aboard an American warship, the Abraham Lincoln, and travels down around the tip of South America and into the Pacific Ocean. After many weeks of searching, the 'monster' is found, and 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 vessel. They are quickly captured and brought inside the vessel, where they meet its enigmatic creator and commander, Captain Nemo. (Nemo means 'no one' in Latin, and may have been inspired by a passage from Homer's Odyssey where Odysseus calls himself Ουτις, which is Greek for 'nobody')

The rest of the story follows the adventures of the protagonists aboard the submarine, the Nautilus, which was built in secrecy and now roams the seas, free of any land-based government. Captain Nemo's motivation is implied to be both a scientific thirst for knowledge, and a desire for revenge on, and self imposed exile from, civilization. Captain Nemo explains that the submarine is electrically powered, and equipped to carry out cutting edge marine biology research. He also tells his new passengers that, while he appreciates having an expert such as Aronnax with whom to converse, they can never leave as he fears they will betray his existence to the world. Aronnax is enthralled by the vistas he is seeing, but Land constantly plots to escape.

Their travels take them to numerous points in the world's oceans, some of them which were known to Jules Verne from real travelers' descriptions and guesses, while others are completely fictional. Thus, the travelers witness the real corals of the Red Sea, the wrecks of the battle of Vigo Bay, the Antarctic ice shelves, and the fictional submerged Atlantis. Back in the Atlantic Ocean, a group of giant squid (cuttlefish in the usual English translation) attacks the Nautilus, and kills a crew member. Nemo is plunged into deep depression, and allows the submarine to wander into an encounter with the Moskstraumen whirlpool off the coast of Norway. This gives the three prisoners an opportunity to escape. They make it back to land alive, but the fate of Captain Nemo and his crew is not revealed.

References

Wikipedia