The narrator, a curious natural philosopher, falls through a cavern into an underground planetary world where he explores cities, courts, religions, and institutions while reporting observations with satirical distance. He describes urban life and manners, examines religious beliefs, political systems, and an academy devoted to learning, undertakes a circumnavigation of the planet, suffers exile into the firmament, and voyages to further fantastic realms. Political upheaval leads to the emergence of a new monarchy and his brief elevation to imperial office before a catastrophic turn returns him to his homeland. The narrative mixes speculative travel, social and philosophical critique, and learned commentary on human institutions.