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!

A botanist has developed a curious flower which only blooms during a full moon and has remarkable properties, instantly healing wounds and giving the strange ability to read other's thoughts. He uses it to rejuvenate his crippled body and sets his sights on his beautiful young assistant.