The narrative begins in the Sahara Desert, where an Englishman encounters David Innes, a man claiming to have spent ten years inside a hollow earth. Innes recounts his past in Connecticut, where he financed the construction of an enormous steel vehicle called the iron mole, invented by an eccentric paleontologist named Abner Perry. Intended to prospect through subterranean rock, the machine malfunctions on its trial run. Instead of boring horizontally to return to the surface, the drill plunges vertically, hurtling downward at terrifying speeds.
As temperatures fluctuate wildly—surging past extreme heat into a bitter subzero ammonia stratum—the explorers brace for death, believing the core to be a molten furnace. To their astonishment, the temperature drops and the craft bursts out of the earth's crust into an interior sky, coming to rest in a weird, tropical landscape. Stepping outside, Innes and Perry realize they have discovered a secondary world resting on the inner surface of a seven-thousand-mile-wide globe, illuminated by a tiny, eternal sun suspended in the absolute center.
This prehistoric interior is populated by colossal reptilian predators, mammoth mammals, and primitive human tribes. Shortly after their arrival, the explorers are attacked by a massive creature resembling a megatherium and are subsequently captured by the Sagoths, a race of gorilla-like gorilla-men who serve the Mahars—a dominant species of telepathic, flying reptiles. Marched across a sun-baked plain in a slave gang, Innes meets Dian the Beautiful, a proud woman from the tribe of Amoz. During the arduous journey to the subterranean city of Phutra, Innes inadvertently commits a grave social blunder by intervening in an unwanted romantic advance without claiming Dian as his mate, alienating her.
Upon reaching Phutra, the prisoners are forced into an arena to witness a brutal combat between prehistoric beasts. In the ensuing chaos, Innes escapes into the surrounding wilderness. He embarks on a solitary odyssey across the inner world, encountering the Mezop fishermen on an inland sea and fighting for survival against colossal amphibians and giant reptiles.
He eventually returns to Phutra to rescue Perry and discovers a secret room containing the Mahars' chemical formula for egg fertilization. Armed with this knowledge, Innes, Perry, and a Sarian warrior named Ghak escape by wearing the hollowed skins of dead Mahars. After a perilous reunion and a long march back to Sari, Innes is elected the first emperor of a federation of human tribes uniting against the Mahars. Seeking advanced knowledge to elevate the Sarians, Innes decides to pilot the iron mole back to the outer crust, bringing Dian with him. Through a treacherous betrayal by the cunning captive Hooja the Sly One, a Mahar is substituted for Dian at the moment of departure. Innes is launched back to the Sahara alone, yearning to find a way back to the inner world and the woman he loves.
