Alien invaders seize Great Mazinger, which had been displayed at the entrance of a museum beside Mazinger Z as a symbol of peace. Grendizer engages the hostile forces, and Great Mazinger is eventually recovered. The Vegan Empire sends a new general to oversee Earth’s conquest; he captures Koji Kabuto and learns of Great Mazinger through interrogation, then takes control of the robot from the museum where it was kept. Now Duke Fleed aboard Grendizer must prevent humanity’s former defender from becoming its own destroyer.