UFO Robot Grendizer vs. Great Mazinger

Alien invaders seize control of Great Mazinger, which stands at the museum entrance beside Mazinger Z as a pair of symbols of peace. Grendizer battles the invaders and rescues Great Mazinger. The Vegan Empire dispatches a new general to oversee Earth's conquest; he swiftly captures Koji Kabuto and, through interrogation, learns about Great Mazinger, then seizes it from the museum where it had been kept as a peaceful emblem. It falls to Duke Fleed aboard Grendizer to stop humanity's former defender from becoming its destroyer.