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 a '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 'Mahors'.