In a galaxy far, far away, the evil Emperor Ming the Merciless is bored. His right hand man, Klytus, offers him a brand new toy. The planet Earth. So Ming has fun creating earthquakes, volcanic eruptions, hurricanes, tidal waves... general climatic chaos really.
Flash Gordon is one of two passengers flying home on a charter airplane (the other passenger is the cute chick & female lead Dale Arden). The plane is struck by lightning and crashes into Dr Zarkov's laboratory along with fragments of moon rock. Now Dr Zarkov has this unbelievable theory that the planet is under attack and he decides to hop into a rocket he happens to have handy, go into orbit and take a look. His assistant refuses so he half-kidnaps Dale and Flash. While unconscious in their rocket, drifting through space they drift too far and end up in Mondo, Ming's universe.
When Ming meets Dale, he instantly decides she will be the next Empress. She's not too keen, perhaps because Ming's marital history makes Henry VIII look positively monogamous (M does have a habit of blasting wives off into space when he gets bored with them).
Anyway, the basic plot is that the trio from Earth have about 24 hours to save their planet (the moon is on a collision course). Dale is kidnapped, Flash is sort of executed but gets revived by Ming's daughter, escapes and plots a coup to overthrow the Emperor. Flash hijacks war rocket Ajax and flies it into the imperial palace during Ming and Dale's wedding, running him through with the pointy spear thing on the ship's nose. Prince Barin, chief cheese in charge of the kingdom of Arboria, is made king of the universe and Prince Vultan, ruler of the Hawk people, is made head of Mondo armed forces. We never find out if Flash, the future Mrs Dale Gordon and Zarkov escape.