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.

Source: Wikipedia

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Same characters: Getter Robo
Parent story: Great Mazinger