A mysterious plague, the Purple Death, ravages the earth. Dr. Zarkov, investigating in his spaceship, finds a ship from planet Mongo seeding the atmosphere with dust. Sure enough, Ming the Merciless is up to his old tricks. So it's back to Mongo for Flash, Dale, and Zarkov.
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Flash Gordon Conquers the Universe (1940) is a sci-fi movie. Starring Buster Crabbe, Carol Hughes, Charles Middleton. Rated 5.8/10 on TMDB. Watch it online free on ZapFlix.