In the Victorian era, scientist Dr. Perry and his assistant David Innes test their Iron Mole, a powerful burrowing machine built to drill through the earth. When the machine tunnels out of control, it carries them into Pellucidar, a strange twilight world deep underground. There, they face a dangerous labyrinth ruled by giant telepathic birds and filled with prehistoric monsters, half-human creatures, cavemen, and man-eating plants.