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.
Un ovni en quête de métal terrestre déploie Gilgigan, un monstre sidéral pour attaquer la ville de Yokoama. L’équipe Getter (trois vaisseaux pouvant se transformer en trois robots différents) se prend un méchant revers, tout comme Antarès et son Great Mazinger qui finit en pièces. Et la rivalité sportive plutôt malvenue entre les pilotes des deux robots n’arrange rien. Seule l’alliance de Great Mazinger et Getter Robot permettrait de vaincre le monstre sidéral, ce dernier ayant la particularité de se réincarner en différents monstres tous plus dangereux les uns que les autres.
Una misteriosa astronave aliena lancia un terrificante mostro dalle sembianze di un rettile con zampe di ragno all'attacco di Yokohama. Il mostro si nutre di metallo e divora qualsiasi struttura per cominciare la propria metamorfosi. La squadra Getta, composta da Ryo, Hayato e Musashi, interviene immediatamente per respingere gli invasori e battere sul tempo il rivale Tetsuya Tsurugi con il suo Grande Mazinga. La competizione tra i due team, però, fa sì che essi si ostacolino a vicenda, ed entrambi i robot riportano gravi danni, mettendo così a repentaglio la sicurezza della Terra. Solo una volta compresa la necessità di collaborare, i giovani piloti riescono a sconfiggere il nuovo nemico, nel frattempo mutato ulteriormente e divenuto un gigantesco droide munito di devastanti armi.
