The main cast from the first serial reprise their roles: Buster Crabbe as Flash Gordon, Jean Rogers as Dale Arden, Frank Shannon as Dr. Alexis Zarkov, Charles B. Middleton as Ming the Merciless, and Richard Alexander as Prince Barin.

Also in the principal cast are Beatrice Roberts as Queen Azura, Donald Kerr as Happy Hapgood, Montague Shaw as the Clay King, and Wheeler Oakman as Ming's chief henchman.

Journeying to the fourth planet, they discover their old enemy from Mongo, Ming the Merciless (Charles B. Middleton), is alive and allied with Azura (Beatrice Roberts), the Witch Queen of Mars.

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.

The evil emperor of Mongo, his Nitron ray destroyed and his escape cut off on all sides by the now hostile Martian forces, is seemingly vanquished by the accidental result of his own machinations and treachery.

Universal then quickly had the feature's title changed to Mars Attacks the World, and a week after the Welles broadcast, opened it at a Broadway theater as a major premiere event.