The following is excerpted from film historian Charles Lee Jackson II's ebook, The Story of the Making of the Flash Gordon Movie Serials, expanded from his classic Filmfax article.
When the Earth finds itself the victim of a mysterious plague, the "Purple Death", Dr. Zarkov, suspecting interplanetary foul play, ascends once again like Little Eva to the heavens, accompanied of course by Flash and Dale. On Mongo, Flash learns that Ming has again seized power, and is indeed responsible for the Purple Death – which kills only intellectuals, leaving the Great Unwashed Masses ready for enslavement.
Zarkov determines that a mineral found only in the frozen north of Mongo can be used to make a cure, and, with Queen Fria's blessing, the adventurers and their new friends rocket to icy Frigia. Without protection (of course invented by Zarkov), Ming's men cannot venture there, so they send "annihilatons", walking bombs, to destroy the good guys.
Ming's forces fail, and Zarkov saves Earth, but Flash and company must return to Mongo to foil additional plans of their enemy. After a sojourn in the Land of the Dead (home of the aboriginal Rock Men), and despite the counter-plots of the evil Sonja, posing as one of Aura's courtiers, Flash eventually corners Ming in his tower. When Captain Sudin, supposedly Ming's trusted guard, locks him in, the emperor is helpless as an explosive-laden rocket homes in on the tower. (A line of dialogue was included to indicate that there was another way out, if only Ming weren't too terror-stricken to think of it – in case a fourth serial was made.)
Patting each other on the backs, the heroes prepare to return home. Told that he's saved the universe, Flash reminds his friends that Ming claimed in his mad ambition to be the universe. So Zarkov decides he'll radio Earth, "Flash Gordon conquers the universe". Wow.