UFO Robot Grendizer vs. Great Mazinger (1976)

Plot

Alien forces infiltrate Earth, seizing control of Great Mazinger, which stands alongside Mazinger Z as a beacon of hope in a museum. Duke Fleed, piloting Grendizer, must confront this new threat. The Vegan empire sends a ruthless general who captures Koji Kabuto and, through interrogation, learns about Great Mazinger's capabilities. He swiftly commandeers the robot from the museum, threatening humanity's safety. With the fate of Earth at stake, Duke Fleed engages in a fierce battle to reclaim Great Mazinger and prevent it from becoming a weapon of destruction.