One snowy night, the Amami couple who has no child encounter a huge white lion-shaped robot, which holds a baby. When they fearfully take that baby, the "lion" suddenly flies away. Though puzzling, Amami couple are happy to accept that baby as their own child.
Eight years later, the time is 2005 A.D. Without notice, a Zonder appear at the Tokyo Bay, merging with debris and a passing ship which have kids on its deck, advancing to Shinjyuku - the center of Tokyo. The Japanese Self-Defense Forces' conventional weapon has no effect and some kids are taken hostage.
But there is the hero who stands against that Zonder Robot. His name is Guy the Cyborg, the commander of GGG's Mechanized Unit. The GGG ("Gutsy Geoid Guard") are the newly-made Japanese secret defense forces against the extraterrestrial invasion.