A renegade sea captain uses a pioneering submarine to force peace on the world.

In San Francisco in 1868, a mysterious beast attacks ships at sea, causing trouble for shippers who cannot find any crew members willing to risk setting sail. Prof. Pierre Aronnax, a famous French marine scientist and scholar, arrives in San Francisco with his aide-de-camp, Conseil, on their way to the Orient. Upon discovering that no ships will sail through the South Seas, Aronnax resigns himself to waiting. After he consents to reporters questioning him about the sea monster that such a beast could possibly exist, he is approached by U.S. Naval official John Howard, who offers him transport to Asia on a warship that will search for signs of the monster. Intrigued, Aronnax accepts, and boards the Abraham Lincoln along with the skeptical commander, Farragut, and brash harpooner Ned Land. The crew is energetic, but after more than three months pass without sighting the beast, morale lags, and Farragut declares the expedition a failure. Just then, they spot a ship being attacked some miles away and rush to the site where, too late to save any of the sailors, they fire cannons upon the attacking "monster." They are helpless against its battering approach, however, and within minutes, the Abraham Lincoln sinks. Aronnax and Conseil manage to stay afloat on a piece of sail, but are soon debilitated from the freezing waters. Finally, they spot a submarine, the Nautilus , and climb aboard, not realizing that it is the "monster" that sank their ship. Below deck, they are shocked to discover a lavish salon, fueled by electric power and richly furnished. Through the massive porthole, Aronnax witnesses an undersea burial attended by men in full diving suits, the likes of which have heretofore been unknown to man. Ned has also managed to find his way to the sub, and upon joining them, insists that they flee in his longboat. Just then, however, the Nautilus 's captain, Nemo, spots the trio inside the ship and instructs his crew to capture them. They are brought before Nemo, who, recognizing Aronnax's name, decides to save him but drown Ned and Conseil. Aronnax surprises Nemo by choosing to die with his comrades rather than share in Nemo's treasures of knowledge, and to test his loyalty, the captain allows the three men nearly to drown before saving them all. He then calls them in to dinner, which consists of such delicacies as unborn octopus and milk of sperm whale. Ned and Conseil are disgusted, but Aronnax appreciates Nemo's ingenious ability to live completely off the fruits of the sea. Nemo then allows them to join a "hunting expedition" to harvest natural materials from the ocean. Ned and Conseil sneak off to explore a sunken ship, upon which they discover buried treasure. When Ned tries to sneak the jewels back onto the ship, they are attacked by a shark and Nemo is compelled to save them. Back onboard, the captain rebukes Ned for bothering with jewels, useless on the Nautilus except in the hold, where they serve as ballast. Later, Aronnax, who is eager to learn all he can from Nemo, scolds Ned and Conseil for angering the captain, but Ned asserts that he wants only to steal some of the treasure and escape from "the madman." Aronnax instructs Conseil to watch over Ned, who is secreting jewels in a homemade guitar when he is spotted by Nemo's barking pet seal, Esmerelda, and must flee. Meanwhile, Nemo, eager to share his discoveries with someone who can appreciate them, reveals his nuclear-powered engine to Aronnax. When the professor urges him to share his miraculous knowledge with the rest of the world, Nemo states that the world is uncivilized and would only use the power for evil. One day, Nemo takes Aronnax ashore on the island of Rorapandi, where he shows him a secret slave camp and reveals that he was once among the men forced to mine minerals to be used for weaponry. He and his crew escaped the island, Nemo explains, and fled to their new home, the isle of Vulcania, where he built the Nautilus and invented all of its machinery. Back on the sub, Nemo confines the trio to their quarters and then, after playing Bach on his organ, attacks and sinks one of the slave ships. Horrified, Aronnax tells Nemo that no inhumanity could be dreadful enough to justify mass murder. Nemo replies that he kills a few slavers to save thousands of future slaves, and reveals that his former government imprisoned him to learn his secrets about nuclear power, eventually torturing his wife and son to death. Soon after, Conseil reads Aronnax's diary and suggests to the professor that he is growing too sympathetic to Nemo. After Aronnax retorts that Nemo is a genius who must be brought back to civilization, Conseil convinces Ned that they must band together in secret to get away. When Conseil tells Ned that the ship is headed to Vulcania, the harpooner schemes to read the navigational plans and form an escape plan. Snooping through the forward deck, they are forced to hide in Nemo's cabin, where Ned befriends Esmerelda and discovers a map painted onto the wall. Later, he throws out Nemo's specimen collection in order to use the bottles to send S.O.S. messages, hording the specimen alcohol for future enjoyment. One day, the Nautilus runs aground on a reef near the shore of New Guinea. When Aronnax refuses to join Nemo ashore, protesting his murderous impulses, Ned and Conseil ask for permission to explore the nearby island. There, they ignore Nemo's warnings and attempt to escape, only to be chased back to the ship by native cannibals. Nemo uses electric currents to repel the natives, then imprisons Ned. Soon after, the crew spots a warship, and because they remain grounded on the reef, the Nautilus is hit, the explosion freeing them from the rocks. Ned's cell is flooded, but he manages to escape while Nemo is forced to submerge the ship farther down than ever before. A giant squid soon attacks the ship, and when Nemo resurfaces the boat to battle the beast, Ned harpoons the squid, saving Nemo's life. Back inside, Ned, horrified at his actions, gets drunk while Nemo sneers at Aronnax's diary depiction of Ned as a hero. The two argue, Aronnax insisting that Nemo still believes in the basic goodness of humanity and the captain dismissing the professor's "gullibility." As the ship soon nears Vulcania, Aronnax has almost convinced Nemo to reveal all his secrets when they are ambushed by warships, responding to Ned's S.O.S. Nemo, desperate to destroy everything on the island before the intruders discover it, goes ashore and sets a nuclear explosion. Upon returning to the ship, he is shot in the back, but manages to steer to safety. In his salon, the dying Nemo announces that this will be the Nautilus 's last dive, and his men prepare to die with him. Aronnax begs him to reconsider, but Nemo merely promises that when the world is ready for his secrets, they will be rediscovered. Ned breaks free from the guards and resurfaces the submarine, allowing him, Aronnax and Conseil to race to a lifeboat. Inside, Nemo collapses, struggling to the porthole to watch Vulcania explode into a mushrooming cloud. As Aronnax, safely on the open sea, watches the Nautilus and its inhabitants sink, he remembers Nemo's assurance that his inventions will one day resurface.