Universal Pictures' second Flash Gordon serial was even more opulent than its first, offering better special effects, more impressive sets, a bigger cast, and a more complicated story. When a mysterious beam of light starts disrupting and destroying the Earth's atmosphere, Flash Gordon (Larry "Buster" Crabbe), Dr. Zarkov (Frank Shannon), and Dale Arden (Jean Rogers) -- accidentally accompanied by wisecracking reporter Happy Hapgood (Donald Kerr) -- swing into action in Zarkov's rocketship, believing that it could be coming from the planet Mongo. Once in space, however, they discover that the ray is originating from Mars. Journeying to the fourth planet, they discover that their old enemy from Mongo, Ming the Merciless (Charles B. Middleton), whom they had believed dead, is still alive, and has formed an alliance with Azura (Beatrice Roberts), the Witch Queen of Mars. From Azura's planet, and under her protection, he is operating a gigantic Nitron ray that is destroying Earth's atmosphere. Azura's powers include the ability to transmute people into figures of living clay, condemned to live in darkened caves, and she is hated and feared by most of the population. Conversely, the Clay People, led by their King (C. Montague Shaw), know the secret of how to eliminate Azura's power, but lack the means of escaping the caves to which their ruined bodies restrict them, in order to battle her. Flash Gordon and his party would seem to hold the answer to their problem, except that the Clay People don't trust him at first, and end up holding Dale Arden hostage. Ultimately the Earth visitors and the Clay People become allies in the tandem quest to defeat Azura and stop Ming from destroying the Earth. Flash, Dale, Zarkov, and Hapgood do battle against Azura's magic and her Martian space-force, Ming's super-scientific weaponry, the treacherous Forest People, and other dangers on the Red Planet. Finally, they win by the classic strategy of divide-and-conquer, showing Azura that Ming has been plotting behind her back to take power from her. Azura's alliance with Ming is broken, at the cost of the Queen's own life, but the Clay People are freed from their curse. And the evil emperor of Mongo, his Nitron ray destroyed and his escape cut off on all sides by the now hostile Martian forces, is finally destroyed by the accidental result of his own machinations and treachery.
by Bruce Eder synopsis