In this 12-chapter serial from Universal Pictures and directors Ford Beebe & Ray Taylor, the evil Ming the Merciless (Charles Middleton) is once again the emperor of the far-off planet of Mongo. He has been sending ships to the Earth to drop "electrified dust" that causes a plague known as the Purple Death. To try and stop him, Flash Gordon (Buster Crabbe), Dale Arden (Carol Hughes), and Dr. Alexis Zarkov (Frank Shannon) travel back to Mongo. There they reteam with ally Prince Barin of Arboria (Roland Drew), but they face fierce foes in Ming, his chief soldier Captain Torch (Don Rowan), and the treacherous Lady Sonja (Anne Gwynne). Also featuring John Hamilton and Chief Yowlachie as the King of the Rock People.
This was the last of the great science fiction serials starring Buster Crabbe, following Flash Gordon (1936), Flash Gordon's Trip to Mars (1938), and Buck Rogers (1939). There's a lot to enjoy here, but things are starting to wear thin, as well. The sets are obviously minor redresses of the ones used in Buck Rogers, and much time is spent in the cramped cockpits of various spaceships. Carol Hughes replaces Jean Rogers (who declined to return), and she does a decent job.
Highlights of the movie include Anne Gwynne as the evil Lady Sonja, a sweet-faced blonde always ready to stab someone in the back. Gwynne is also part of a long SF legacy, as besides her appearance in this, her grandson is Chris Pine, the most recent Captain Kirk in the three latest Star Trek movies. I also liked the Rock People, who live in a desolate rocky waste known as the Land of the Dead. They wear silly rock outfits, and their spoken language is English played backward. They also get menaced by a giant lizard in footage that looks lifted from another movie. Speaking of which, there are both scenes and music borrowed from the German movie White Hell of Pitz Palu (1930).
As I said, this marked the end of the science fiction serial for the most part, as the format leaned more heavily on superhero, police, and spy serials during the forties, with a brief return to science fiction in the early 50's before the format was permanently killed by TV.