When a UFO appears over Japan, both the heroes of Great Mazinger and Getter Robo want to investigate it first, out of a sense of rivalry. Getter finds it first since the Mazinger robot is being repaired. However they are unable to defeat the apparently living vehicle. It had already dropped a metal-eating monster on Japan named Gilgilgan and Great Mazinger confronts it only to be badly beaten. Later, despite the robot pilots' protests, their two bases decide to join forces to stop Gilgilgan as he starts becoming more powerful and begins to change. They make a plan to lure it to an empty island where they can battle it without causing collateral damage. They get Boss to lure the monster there where it eats Boss Borot in the process. The combined heroes almost defeat Gilgilgan until the UFO returns and allows itself to be eaten to cause its own creation to grow to its final form. During the intense battle the heroes manage to find its weak points and destroy Gilgilgan. Afterward they promise to be friends from then on.

Theatrical feature. 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 comandeers it from a museum where it was kept as a symbol of peace. It is now up to Duke Fleed and Grendizer to stop humanity's former defender from becoming its destroyer.

A giant sea monster known as "Dragonsaurus" has emerged from out of nowhere, terrorizing the depths of the oceans. Great Mazinger, Grendizer and the Getter Robo G team join forces to combat the new threat. But their task becomes more complicated when Dragonsaurus swallows up Boss Borot and makes its way towards Tokyo.