Twenty Thousand Leagues Under the Seas is a classic science-fiction novel by French author Jules Verne. First serialized in 1870, the plot is straightforward. There is believed to be a monster terrorizing ships on the oceans of the world and an expedition is organized to hunt it and eradicate it from the seas. At the first encounter, marine biologist M. Arronax and his servant Conseil along with a Canadian harpooner (Ned Land) are washed overboard and are picked up by the submarine vessel Nautilus, which is the actual perpetrator of the terrorizing incidents. The Nautilus is the brainchild and product of Captain Nemo who keeps them on board as virtual prisoners, seemingly without a chance of release. This is the story of their journey on board the Nautilus. The novel was first translated into English in 1873 by Revd. Lewis Page Mercier, who made a swathe of translating errors, some of which are addressed in this version (including mistranslating the title as "under the Sea," rather than "under the Seas," "contrivances" rather than "devices," and "cork-jacket" as opposed to "diving suit" and others). However, Mercier's text became the standard English translation and forms the basis of this text.

The title refers to the distance traveled under the various seas, not the depth achieved: a 20,000-league journey. Also, the hunting of whales was a simple fact of life in Vernes's time and he refers to it in a matter-of-fact way which can seem barbaric to the modern listener, but is not intentionally cruel.