When the Vegan empire dispatches a new general to oversee the takeover of Earth, he quickly manages to capture Koji Kabuto and, learning of Great Mazinger through interrogation, quickly commandeers it from a museum where it was kept as a symbol of peace. It is now up to Duke Fleed aboard Grendizer to stop humanity's former defender from becoming its destroyer.
When a mysterious gigantic monster, called Dragosaur, appears from the depth of the oceans and threatens the world, a team of super-robots from Japan is activated and sent out to defeat it: Grendizer, The Great Mazinger and Getter G.