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.

太空火箭在发射时爆炸，不明飞行物目击报告的增加，最终导致一个死亡的银河系外文明的外星幸存者直接试图使用大规模杀伤性射线武器从不透水的飞碟入侵地球。