Alien invaders take over Great Mazinger who is at the entrance of a museum next to Mazinger Z as symbols of peace. Grendizer fights the evil invaders and Great Mazinger is recovered. 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.
Unfortunately the movie UFO Robot Grendizer vs. Great Mazinger is not yet available on HBO Max.
| Production | Chiaki Imada | Producer |
| Directing | Osamu Kasai | Director |
| Editing | Chikako Matsubara | Editor |
| Writing | Keisuke Fujikawa | Screenplay |
| Visual Effects | Yuki Kinoshita | Key Animation |
| Directing | Johei Matsuura | Assistant Director |
| Writing | Go Nagai | Original Series Creator |
