Prof. Pierre Arronax was a French marine biologist who served as the Assistant Professor of Natural History at the Museum of Paris. Sometime in 1866, he and his assistant Conseil were in the United States when they were approached by Secretary of the Navy J.B. Hobson. Hobson asked for Arronax's assistance in identifying a sea monster which had been sinking ships throughout the world's oceans. Arronax accepted his request, and he and Conseil soon found themselves aboard the Navy frigate Abraham Lincoln in a hunt for the monster. Judging from the general description of it, Arronax theorized it to be a giant narwhal.
The creature however turned out to be anything but. Instead, to the surprise of everyone concerned, it was actually a submarine boat! When fired on by the Abraham Lincoln, the submarine retaliated by ramming the frigate, damaging its rudder. Arronax fell overboard, and was saved from drowning by Conseil, who leaped in after him. However the two were stranded in the middle of the ocean as the crippled Lincoln drifted away from them. Eventually however Arronax and his friend stumbled across the very submarine which had attacked them, and, along with harpooner Ned Land, who had also fallen overboard from the Lincoln, they climbed aboard and were immediately taken prisoner by the vessel's captain and crew.
Arronax soon learned the submarine's name was the Nautilus and its captain had no name, at least none he would willingly give. Instead he sufficed to introduce himself as Captain Nemo. having read Arronax's works on marine biology, Nemo, an admirer of the Frenchman, spared the lives of Arronax and his fellow captives, but under the condition they never, ever leave the Nautilus, and always do exactly as commanded. Arronax agreed to these conditions.