The men of Earth are stricken with a mysterious disease that causes instant death and leaves a purple mark on the victim's forehead. Dr Zarkov discovers a spaceship from the planet Mongo and realizes this is the source of the plague. Flash Gordon, Dr Zarkov, and Dale Arden chase off the ship and head to Arboria to enlist their help. They learn of an antidote but getting it will be no easy task, going up against an army of humanoid robots controlled by Ming the Merciless