Four adventurers become trapped in Magnet Mountain's abandoned Castle of Satan. To escape the citadel's strange and terrible demonic attacks, they flee into a deep underground labyrinth. Here they discover a vast subterranean kingdom peopled by bat-winged beasts & devils whose hierarchy is headed by King Satan himself! To save their souls the adventurers must assist Satan's minions to infiltrate the mortal world above - but a clever stratagem foils the plan!