There is a sinister and monstrous narwhal beneath the ocean; this is confirmed independently by ships from a number of different nations. The United States government quickly puts together an expedition of experts who they believe will be able to hunt down and kill the monster. The expedition is to be led by renowned Canadian whaler Ned Land, who is also a master harpoonist. He is joined by Professor Pierre Aronnax, a French marine biologist who is added to the expedition at the last minute when it is learned that he is in New York City for personal reasons. His faithful bagman, Consell, will travel too.
The expedition leaves aboard the USS Abraham Lincoln, departing Brooklyn for the Pacific Ocean via the famously treacherous waters of Cape Horn. The monster is located and attacked, but it fights back, damaging the rudder of the ship and throwing the men into the water They cling on to what they think is the fin or the tail of the narwhal but find it to be neither; the monster they are hunting is a submarine - a very futuristic submarine, the like of which they have never seen before. The men are captured and brought before the submarine's captain, Nemo, who is not only captaining the craft but also designed and built it too.
At first, the men reach a copacetic state of grace. They are all intrigued by life under the water and all have a desire to learn as much as they can, which makes their time on the Nautilus rather enjoyable. Nemo is a scientist himself and also eager for knowledge, but he also wants to avenge the deaths of his wife and child who were killed by the government of his country. He is enthusiastic about his submarine and is happy to explain its intricacies at great length. It is powered by electricity and can also undertake advanced research. Aronnax and his assistant are enthralled by it all, but Ned Land feels uneasy and full of concern. All seems to going well, until Nemo refuses to let the men leave. He is genuinely pleased to be in the presence of Aronnax, a man whose work his is familiar with, and whom he respects, but his work is secret, and now that he has revealed it to them, cannot allow them to get away.
Ned Land cannot think of anything else other than escaping the vessel. This completely consumes him and so he does not experience anything positive about their explorations without borders under the sea. Although uneasy about their situation, Aronnax and Conseil still gain enjoyment and fascination about their exploration. They roam their new undersea world, taking in the corals of the Red Sea, Antarctic ice shelves, shipwrecks that sank after the Battle of Vigo Bay. They find the legendary kingdom of Atlantis (yes, it really is there), use underwater air guns to go fishing, and hunt sharks. They hold an underwater funeral for a crew member who is killed by some unknown attack on the Nautilus. Ned Land, Aronnax, and Conseil were incapacitated at the time.
Nemo is initially polite and friendly towards Aronnax, in part because he admires his work so genuinely; however, his demeanor suddenly changes and avoids him, which causes Aronnax to feel uneasy and finally come to understand how Ned Land has been feeling all along. When the Nautilus is attacked by a ship from the very nation that is responsible for the murders of Nemo's wife and child, Nemo finally loses his mind. He ignores Aronnax's pleas for him to be merciful; living up to his nickname "angel of hatred" he destroys the enemy ship and causes it to sink to the bottom of the ocean. Aronnax is horrified. Nemo's spirits plunge as low and as deep as the ship he has just annihilated. He withdraws. The Nautllus becomes a submarine with no captain, roaming randomly, Ned Land feels ever more pessimistic but sees an opportunity for the men to escape. Aronnax is conflicted because he wants to escape just as much as Land does, but he also wants to see Nemo one more time before he does. However, if Nemo finds out that he is planning to leave the Nautilus he will never let him go. Glimpsing Nemo one night, when the captain does not know that he is doing so, he sees a deeply troubled man, praying for deliverance. Aronnax tells the men it is time for them to leave. They loosen the submarine's dinghy but do not realize that they are now in waters known as the Maelstrom, strangely spinning and hard to navigate. They are washed up on an island off the Norwegian coast. They never see the Nautilus again.
