An officer accused of cowardice volunteers to bring back General Custers's body after Little Big Horn.

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.

While driving through the desert with his wife Carol Marvin to a military base to send the eleventh rocket to the orbit of Earth for helping the exploration of the outer space in the Operation Sky Hook, Dr. Russell A. Marvin and Carol see a flying saucer and accidentally records a message in their tape recorder. Once in the base, Dr. Russell is informed by his father-in-law and general that the ten first satellites had mysteriously felt on Earth. When Dr. Russell decodes the message, he encounters the aliens that ask him to schedule a meeting with the leaders of Earth in Washington in 56 days with the intention to invade Earth without panicking the population. Dr. Russell develops an anti-magnetic weapon that becomes that last hope of human race against the hostile aliens.