Twenty Thousand Leagues under the Sea

In 1866, ships of various nationalities sight a mysterious sea monster which is speculated to be a gigantic narwhal. The United States government assembles an expedition in New York City to find and destroy the monster. Professor Pierre Aronnax, a French marine biologist and the story's narrator, is in town at the time and receives a last-minute invitation from the government to join the expedition. A Canadian whaler and master harpooner named Ned Land and Aronnax's faithful manservant, Conseil, are also among the participants.
The expedition leaves Brooklyn aboard the US Navy frigate Abraham Lincoln, captained by Admiral Farragut. The ship travels south around Cape Horn into the Pacific Ocean. After a five-month search ending off Japan, it locates and attacks the monster, which damages the ship's rudder. Aronnax and Land are hurled into the sea, and Conseil jumps in after them. They survive by climbing onto the "monster", which, much to their surprise, is in fact a futuristic submarine. They wait on the deck until morning, when they are caught and introduced to its mysterious constructor and commander, Captain Nemo.
The rest of the novel describes the protagonists' adventures aboard the submarine Nautilus, which was built in secrecy and now roams the seas, beyond the reach of governments. In self-imposed exile, Captain Nemo seems to have a dual motivation—a quest for scientific knowledge and a desire to escape terrestrial civilisation. Nemo explains that his submarine is electrically powered and can conduct advanced marine research; he also tells his new passengers that his secret existence means he cannot let them leave; they must remain on board permanently.
They visit many oceanic regions, some real and others fictional. The travellers view coral formations, sunken vessels from the Battle of Vigo Bay, the Antarctic ice barrier, the transatlantic telegraph cable and the legendary underwater realm of Atlantis. They travel to the South Pole and are trapped in an upheaval of an iceberg on the way back, caught in a narrow gallery of ice from which they are forced to dig themselves out. The passengers also put on diving suits, hunt sharks and other marine fauna with air guns in the underwater forests of Crespo Island and attend an undersea funeral for a crewman who died during a mysterious collision experienced by Nautilus. When the submarine returns to the Atlantic Ocean, a school of giant squid ("devilfish") attack it and kill another crewman.
The later pages suggest Captain Nemo went into undersea exile after his homeland was conquered and his family were slaughtered by a powerful imperialist nation. Following the episode of the devilfish, Nemo largely avoids Aronnax, who begins to side with Ned Land. Ultimately, Nautilus is attacked by a warship from the mysterious nation that has caused Nemo such suffering. Carrying out his quest for revenge, Nemo—whom Aronnax dubs an "archangel of hatred"—rams the ship below its waterline and sends it to the bottom, much to the professor's horror. Afterwards, Nemo kneels before a portrait of his deceased wife and children, then sinks into a deep depression.
Circumstances aboard the submarine change drastically: watches are no longer kept, and the vessel wanders about aimlessly. Ned becomes so reclusive that Conseil fears for his wellbeing. One morning, he announces that they are in sight of land and have a chance to escape. Aronnax is more than ready to leave Captain Nemo, who now horrifies him, yet he is still drawn to the man. Fearing that Nemo's very presence could weaken his resolve, he avoids contact with him. Before their departure, the professor eavesdrops on Nemo and overhears him calling out in anguish, "O almighty God! Enough! Enough!" Aronnax immediately joins his companions as they carry out their escape plans, but as they board the submarine's skiff they realise Nautilus has seemingly blundered into the ocean's deadliest whirlpool, the Moskstraumen (more commonly known as the Maelstrom). They escape and find refuge on an island off the coast of Norway. The submarine's ultimate fate remained unknown until the events of The Mysterious Island (1875).