As alien invaders conquer Great Mazinger, positioned alongside Mazinger Z as symbols of peace at a museum entrance, Grendizer steps in to battle the malevolent forces and reclaim Great Mazinger. However, when the Vegan empire sends a new general to Earth to facilitate its conquest, Koji Kabuto is captured, leading to the enemy seizing Great Mazinger from its museum display. Now, Duke Fleed aboard Grendizer must prevent humanity's once protector from turning into its adversary.