The narrative begins in 1665 with Niels Klim, a recently graduated Norwegian student of natural philosophy whose insatiable curiosity leads him to explore a mysterious, murmuring cave on Floïen Mountain. During his descent, the rope snaps, plunging him into total darkness before he emerges into a wondrous inner world illuminated by a lesser central sun. Klim discovers that he is revolving as a satellite around a strange subterranean planet before finally landing on its surface. There, he encounters a race of rational, speaking trees whose culture, laws, and social structures at first seem absurdly inverted to his European eyes, but later reveal profound layers of wisdom, particularly regarding the equality of the sexes and the selection of rulers based on true merit rather than birth or quickness of wit.
After serving as a court messenger and traversing various provinces of the Potuan Empire, Klim compiles his observations and sets out to explore the rest of the hollow globe, known as Nazar. His journey takes him through numerous bizarre principalities, each governed by extreme social experiments or physical oddities. He visits lands where inhabitants are entirely devoid of diseases, provinces where the aged behave like impulsive children, and cities ruled by women or obsessed with academic disputations. He encounters societies of philosophers whose extreme intellect leads to absurdity, regions where the inhabitants are blind to near objects, and utopian territories governed purely by innate innocence. Each stop exposes the structural flaws of human civilization, contrasting European vanities and hypocrisies with alternative societal systems.
Continuing his travels, Klim reaches the grand empire of Mezendoria, where animals and trees share civic duties according to their natural aptitudes, creating a remarkably harmonious administration. Following this, he undertakes a voyage across the subterranean seas, encountering singing syrens, custom-inspecting jack-daws, musical instrument inhabitants, and the surreal wonders of the Mezendorian islands. These adventures expand his understanding of governance, human nature, and the sheer diversity of rational existence within the concave structure of the Earth.
Returning to the Potuan capital, Klim attempts to introduce reforms inspired by his travels, particularly aimed at curbing female political authority. The initiative backfires, leading to his trial and subsequent banishment to the firmament above the subterranean world. Transported by giant post-birds, Klim arrives in Martinia, a realm inhabited by talkative, vain monkey-like creatures governed by an aristocracy of frivolous intellectuals. Here, he achieves sudden fame and nobility by introducing the European invention of perriwigs, only to fall from grace after rejecting the romantic advances of the syndick's wife.
Condemned to galley slavery, Klim eventually survives a catastrophic shipwreck that destroys his vessel during a violent storm. Washed ashore on an uncharted coast, he discovers an isolated population of humans whose primitive lifestyle closely resembles that of early mankind. Embraced by these simple people as an ambassador from the sun, Klim spends his remaining years helping them develop their understanding, ultimately finding contentment and purpose far away from the complexities and absurdities of the civilized world he left behind.
