 A mysterious plague, the Purple Death, ravages the earth. Dr. Zarkov, investigating in his spaceship, finds a ship from planet Mongo seeding the atmosphere with dust. Sure enough, Ming the Merciless is up to his old tricks. So it s back to Mongo for Flash, Dale, and Zarkov, this time with ready-made allies waiting: Prince Barin of Arboria and Queen Fria of the frozen northern land of Frigia; where, it so happens, is found polarite, antidote to the plague. But Ming will use all his forces to keep our heroes from thwarting his plans of conquest...