Amid the political chaos sweeping across the world in 1939, a new terror arises -- the Purple Death -- and people around the world succumb at random by the hundreds, then thousands, with the identifying symptom being a purple spot on the victim. The authorities are baffled as to the cause or the treatment, and panic is spreading. Dr. Alexis Zarkov (Frank Shannon) determines that the Purple Death is linked to extraterrestrial events. Along with Flash Gordon (Larry "Buster" Crabbe) and Dale Arden (Carolyn Hughes), Zarkov finds an alien spaceship, which they recognize as being from the planet Mongo, home of their old enemy, Ming the Merciless, spreading some sort of dust in the Earth's upper atmosphere. Flash, Dale, and Zarkov head for Mongo, where they discover that Emperor Ming (Charles B. Middleton), whom they believed had been killed at the end of their battle with him on Mars (told in Flash Gordon's Trip to Mars), is still alive. He is threatening not only to conquer all of Mongo, but is attacking Earth again, using a "Death Dust" spread by his spaceships that will eventually destroy everyone on Earth.
Flash, Dale, and Zarkov form an alliance with their old friend, Prince Barin (Roland Drew), the rightful ruler of Mongo, who with his wife, Princess Aura (Shirley Deane) -- Ming's own daughter -- rules the peaceful kingdom of Arboria, resisting Ming's military might with their small fleet of ships, the aid of neighboring free kingdoms, and the help of a tiny handful of officers within Ming's own palace who remain loyal to the prince. Their first task is to secure a neutralizing agent for the Death Dust, which exists in the frozen northern kingdom of Frigia, but before they can do that, they have to free the imprisoned Frigian military leader General Lupi (Ben Taggart), who has been captured by Ming. Flash rescues the general, who is about to be used as the subject of a scientific experiment, and secures the aid and gratitude of the Frigians. This barely slows Ming in his plans for conquest, however, and over the next 11 chapters, Flash Gordon and his friends and allies -- including Ronal (Donald Curtis), Roka (Lee Powell), and Captain Suden (William Royle) -- take their battle for the safety of the Earth and the freedom of Mongo to the far reaches of the planet. Battling Ming and his villainous henchmen -- including Captain Torch (Don Rowan) and Lady Sonja (Anne Gwynne) -- from Mongo's frozen northern wastes to its uncharted deserts, Flash and his allies outmaneuver and generally outfight and outwit Ming's larger, better equipped army and spaceship fleet, but they are nearly undone by the spies that Ming has placed in Barin's own household. The bravery of the Earth hero and his friends, and the patriotism and sacrifices of Mongo's people ultimately prove too much for the evil emperor, who finally faces impending destruction from one of his own fiendish inventions.