Dr. Abner Perry (Peter Cushing), a British Victorian scientist, and his US financier David Innes (Doug McClure) make a test run of their Iron Mole drilling machine in a Welsh mountain, but end up in a strange underground labyrinth ruled by a species of giant telepathic flying reptiles, the Mahars, and full of prehistoric beasts and cavemen. They are captured by the Mahars, who keep primitive humans as their slaves through mind control. David falls for the beautiful slave girl Princess Dia (Caroline Munro) but when she is chosen as a sacrificial victim in the Mahar city, David and Perry must rally the surviving human slaves to rebel and not only save her but also win their freedom.