A group of wealthy yachtsmen find a copper cylinder bobbing on the ocean waves. Inside it they discover a manuscript giving the account of the extensive travels in the Antarctic of a sailor named Adam More. Adrift on the ocean, he and his companion Agnew had landed on an island only to fall into the clutches of cannibals.
More escaped, but Agnew did not. His boat was drawn to a distant land where it plunged into a vast tunnel and continued through caverns and chasms. There he encountered such hazards as sea monsters and active volcanoes. Eventually he met a lost race, the Kosekin, whose philosophy embraced darkness, poverty and death.
He fell in love with a beautiful young woman called Almah, and they tried to escape, but were caught by a rival for her affections and told that they would have the starring roles in a human sacrifice, the greatest honor possible in that society.
As in many books of the time, the adventures and the language are sensationalist, with danger piled upon danger. However, in the passage where the yachtsmen try to rationalize More's story as if dissecting a scientific paper, De Mille is poking fun at himself and his story. There is however a moral lurking in the background: the absurdity of the Kosekin's philosophy reflects late Victorian concerns about the declining role of God in society.
