The main story of the novel is the narrative of the adventures of Adam More, a British sailor shipwrecked on a homeward voyage from Tasmania. After passing through a tunnel of volcanic origin, he finds himself in a "lost world" of prehistoric animals, plants and people sustained by volcanic heat despite the long Antarctic night. In this strange volcanic world, More finds a well-developed human society that, in the tradition of topsy-turvy worlds of folklore and satire (compare Sir Thomas More's Utopia (1516), Erewhon (1872) by Samuel Butler, or Charlotte Perkins Gilman's Herland (1915)) has reversed the values of the Victorian era. Wealth is scorned and poverty is revered, death and darkness are preferred to life and light, and, rather than accumulating wealth, the natives seek to divest themselves of it as quickly as possible.
A secondary plot and framing device concerns four yachtsmen who find Adam More's manuscript sealed in a copper cylinder. They comment on More's report, and one of them identifies the Kosekin language as a Semitic language, possibly derived from Hebrew.
