The Mysterians (ミステリアン, Misuterian?) is a humanoid species that thousands of years ago lived on an asteroid between Mars and Jupiter. They were far more scientifically advanced than the beings of Earth. When Earth entered the Neolithic Age, this alien race had discovered the atom bomb. Despite all this progress, their world was devastated by terrible weapons when war erupted among their people. The bodies of those who survived were mutated and contaminated with Strontium-90, causing roughly 80% of the new born population to be horribly disfiguredand following this catastrophe, the survivors moved to Mars. However, since that planet was such a wasteland, they had to find a new home that could fit all of their needs. They desired Earth...
Hiding on the dark side of the moon, these people planned their invasion for years. Finally, in the Earth year 1957, they found a place where they could begin their attack: Japan. The aliens arrived on Earth undetected, by transporting their UFOs from their mother ship and hiding underground in the Mt. Fuji area. They used a giant robotic machien called Moguera as the tool of their invasion but failed.
In 1965, a similar species calling itslef the Natarls attacked Earth with a wide array of technology and and ingenuity in their attempt to overthrow the Earth by various means, including mind control, sabotage, anti-gravity rays, space-torpedoes, saucer attacks, and finally their huge Mothership.
The Natarls are eventually defeated through the cooperation of Earth's nations in developing new weapons, launching a fleet of advanced rocket ships, and a sneak-attack on the aliens moon base.