The author relates how, traveling the Sahara, he's encountered a remarkable vehicle & pilot, David Innes, a man with a remarkable story to tell. Innes is a mining heir who finances an experimental "iron mole," an excavating vehicle designed by his elderly inventor friend Abner Perry. In a test, they discover the vehicle cannot be turned. It burrows 500 miles into the Earth, emerging into the unknown interior world of Pellucidar. In Burroughs' concept, Earth is a hollow shell with Pellucidar as the internal surface of that shell. Pellucidar is inhabited by prehistoric creatures of all geological eras, & dominated by the Mahars, a species of flying reptile both intelligent & civilized, but which enslaves & preys on local stone-age humans. Innes & Perry are captured by the Mahars' ape-like Sagoth servants & taken with other human captives to the chief Mahar city of Phutra. Among their fellow captives are the brave Ghak, the Hairy One, from the country of Sari, the shifty Hooja the Sly One & Dian the Beautiful of Amoz. Innes, attracted to Dian the Beautiful, defends her against the unwanted attentions of Hooja, but due to his ignorance of local customs she assumes he wants her as a slave, not friend or lover, & subsequently snubs him. Only later, after Hooja slips their captors in a dark tunnel & forces Dian to leave with him, does David learn from Ghak the cause of the misunderstanding.