David Innes is a young man who has just inherited a large mining company. An eccentric inventor, Abner Perry, convinces Innes to underwrite a project to build an 'iron mole', claiming it will make them both wealthy. The mechanical beast works well, actually too well. On the maiden voyage, instead of digging for a few minutes and returning, they plunge straight through the earth's crust into the 'inner world' of Pellucidar. This world resembles earth but is a horizon-less, primeval tropical landscape where the sun neither sets nor rises, and is populated by 'Sagoth' gorilla men, wild human slaves, and the ruling hypnotic reptilian 'Mahars'.

Upon arrival at this strange world, the men are immediately captured and enslaved. But soon Perry learns to read the language of the Mahars, and discovers a secret way to turn the tables! True to Burroughs form, this non-stop fantasy thriller weaves together savage islanders, pterodactyls, telepathy, and, of course, romance.