In 1866, the world is captivated by rumors of a “phosphorescent” sea monster that is spotted by several ships around the globe. The monster is depicted in newspaper articles, songs, and plays. After the monster bores a large whole inside the bottom of a Quebecois passenger ship, all unsolved shipwrecks are blamed on the mysterious creature.
The narrator, a professor of natural history from Paris named Pierre Arronax, has just returned to New York after six months of fieldwork in Nebraska. He is the author of a book entitled Mysteries of the Unsounded Depths Undersea, and is thus consulted as an expert on what the mysterious monster could possibly be. He argues that it is likely some kind of gigantic narwhal. He is invited to join an expedition on a U.S. naval ship, the Abraham Lincoln, in search of the monster, and enthusiastically accepts. He is accompanied by his faithful servant, a Flemish man named Conseil. The ship is commandeered by Captain Farragut, who considers it his personal mission to find and destroy the monster. Farragut offers $2,000 to whomever is the first person aboard to catch sight of it. One of the other men on board is an expert harpooner named Ned Land, a 40-year-old from Quebec. Although they have very different characters, Ned and Arronax bond over their connected countries of origin.
After a long period searching in vain, Farragut announces that the mission will be called off if the monster isn’t found in the next three days. However, on the final day, Ned sees the monster glowing brightly in the water. The Abraham Lincoln initially retreats, but after the monster appears to full asleep, it advances and attacks. Ned harpoons it and at this point jets of water erupt from it, throwing Arronax into the ocean. Arronax and Conseil almost drown. Arronax eventually loses consciousness, and when he wakes up he is on board the monster—which is actually not a monster at all, but a submarine. Ned and Conseil are there too, and the three of them are taken down into a prison cell by men wearing masks. A tall man and his shorter companion enter the cell, and speak to each other in a language Arronax doesn’t recognize. The captives try to introduce themselves in English, French, German, and Latin, but none of these attempts elicit a reaction.
After Arronax falls asleep and wakes up again, the tall man from earlier—the submarine’s commander, Captain Nemo—introduces himself in perfect, unaccented French. He explains that he has taken the men captive as prisoners of war. Nemo himself has fled society and its “stupid laws.” He is an admirer of Arronax’s work on the deep sea, and looks forward to showing Arronax the “fairyland of marvels” that can be found in the ocean. Nemo shows Arronax an impressive museum and library filled with an enormous number of items. He also explains that the submarine, the Nautilus, is powered by electricity. Nemo then tells Arronax that he and the others will remain on the vessel for the rest of their lives as prisoners.
Nemo and Arronax go to smoke seaweed cigars in the vessel’s saloon, and Nemo explains that is an engineer who built the submarine himself in secret. He also reveals that he is extraordinarily wealthy. The Nautilus periodically goes up to the water’s surface to stock up on oxygen before descending back down to the ocean floor. One day, Nemo shows Arronax, Ned, and Conseil oxygen tanks which can be worn underwater, and they go on a hunting expedition to the underwater “forests of the island of Crespo.” During the expedition, two man-eating sharks pass by, but fortunately do not notice the men swimming.
Christmas passes, although there are no celebrations on the ship. On the first day of 1868 Conseil wishes Arronax a happy New Year, and Arronax wonders if their captivity will soon end. Shortly after, the Nautilus runs aground on an island in the Torres Strait, and Nemo gives the three captives permission to explore the island. They are ecstatic to be on dry land and excited by the prospect of hunting and eating red meat. They discover an amazing array of wildlife and greedily hunt birds and mammals, but are then driven off the island by native Papuans who shoot stones at them. Shortly after, the tide rises enough to push the Nautilus off its beached position.
Over time, the captives become used to their rather repetitive life on board the Nautilus. At one point, Nemo is disturbed by something he sees through his telescope and sends the three men downstairs, putting them to sleep by drugging their food. The next day, Nemo asks Arronax to provide medical treatment to one of his sailors, who has a terrible head wound. Arronax cannot save him, and he dies, which devastates Nemo.
After passing through the Bay of Bengal, the ship arrives on the island of Ceylon, which is known for its pearl fisheries. The men go on a pearl-fishing expedition, during which Nemo risks his life in order to save another pearl diver—whom he doesn’t know—from being eaten by a shark. Not long after, the Nautilus arrives in the Mediterranean, and this prompts the three captives to discuss the possibility of escape. Arronax worries that if they make an escape attempt and it fails, they may ruin their chances of freedom forever. However, in the end the submarine races through the Mediterranean, such that the captives don’t get a chance to escape. Before long, they are back in the Atlantic Ocean.
Determined to escape anyway, Ned makes a plan, and Arronax feels conflicted. He wants his companions to be free but is devastated by the prospect of leaving the Nautilus and the unique opportunities for research it presents. While having a conversation with Nemo, Arronax realizes that Nemo has gained his wealth by raiding the booties of shipwrecks and selling them. Arronax asks Nemo if it wouldn’t be better to share these treasures with the world, but Nemo indignantly replies by insisting that he acts in solidarity with “the oppressed people of this world.” The submarine has moved too far away from Europe, however, and Ned is forced to abandon his escape plans, which secretly relieves Arronax. Arronax considers bringing up the prospect of his freedom with Nemo, but is worried the conversation won’t go well. Lately Nemo has become increasingly withdrawn, and seems angry. The ship travels down to the South Pole, moving through waters riddled with icebergs. Nemo proposes the idea of travelling under the Great Ice Barrier, a risky move. However, after talking to Arronax, Nemo decides to go through with it.
Using scientific instruments to test the angle of the light, Nemo and the crew are able to determine that they have made it to the South Pole. Nemo immediately declares that he has “taken possession” of the territory. As they move away from the area, the Nautilus gets hit by a falling iceberg that ends up trapping it inside a cage of solid ice. With detached calmness, Nemo reveals that they only have two days’ air left until they suffocate. The men get to work attempting to hack their way through the ice. It is the most difficult labor of Arronax’s life, and it seems certain that it won’t work. However, just as the men on board the Nautilus begin to feel the effects of the oxygen running out, Nemo has the idea of blasting the ice around the submarine with boiling water. Miraculously, this works, although Arronax passes out before they make it through. Someone carries him up to the submarine’s platform, where he takes in huge gulps of fresh air.
After this harrowing incident, the ship journeys up the Atlantic past the Latin American continent, and ends up in the Bahamas, where it is attacked by a giant squid. One sailor is killed in the squid’s grip, which worsens Nemo’s depression. Arronax is also profoundly disturbed by the sailor’s death. Ned is so desperate for freedom that he is on the verge of a breakdown. Nemo reveals that he has written a manuscript about his “knowledge of the sea,” which also contains his life story. Arronax hopes this means that Nemo plans to end his exile at some point in order to share the book with the world. He brings up the prospect of his freedom, but Nemo reacts with fury. At this moment, the submarine is caught in a terrible storm.
After being tossed away from the North American coast, the submarine floats along until they arrive at a destination that Nemo announces with a solemn acknowledgment. He describes a shipwreck at this location, and in reverent tones says the ship’s name was the Avenger. Arronax realizes that Nemo is motivated by a fierce anger and desire for revenge. In the distance, Arronax, Conseil, and Ned spot a warship, although they can’t tell which country it comes from. They hope to escape on it, but realize that the ship sees the Nautilus as an enemy.
Nemo declares that he is “the oppressed” and the mysterious ship is the “oppressor.” He ends up attacking the ship from beneath, destroying it and killing all those on board. He then runs into his bedroom and stands in front of a portrait of a woman and two children, sobbing hysterically. Arronax is horrified, and feels like he is losing his grip on reality. Around two weeks later, Ned once again announces that it is time for them to make their escape. They sneak out at night, but are almost intercepted by Nemo. However, at that moment the submarine becomes caught in an enormous whirlpool near Norway, which is infamous for never leaving any survivors.
Arronax loses consciousness and wakes up in a fisherman’s hut on a Norwegian island, accompanied by Ned and Conseil. He never finds out what happens to Nemo or the Nautilus, though he hopes that the anger in Nemo’s soul has subsided. Despite everything, he still believes that he and Nemo share a special connection based on their profound love for, and understanding of, the sea.
