After Koji Kabuto and Sayaka Yumi leave to the USA, it's up to the rash orphan Tetsuya Tsurugi to pilot the next Mazinger robot: the Great Mazinger. Under the direction of Dr. Kenzo Kabuto and helped by his adoptive sister Jun Honou, Tetsuya battles the Miccene Empire.

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.

When a UFO arrives from space and attacks the Getter Institute, Getter Robo is launched to stop it; but the might of the UFO and its giant space monsters prove to be too much for the Getter team. Not only is Getter Robo defeated, but Ouishan is killed in action! With Getter Robo out of commission, the world's only hope appears to be Great Mazinger, but the aliens' power might also be too much for even him. Can Dr. Saotome build a new Getter Robo in time to help save Great Mazinger, Japan, and the entire world?

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. Source: ANN

When an UFO delivers a metal-eating monster to the heart of Japan, Getter Robo and Mazinger Z are launched to combat the alien menace. Fighting individually, the space monster's metal-eating and acid-spitting abilities prove too much for Japan's super robot forces - only their combined might stands a chance against defeating it! But with Mazinger Z damaged, can Getter Robo defend against the monster long enough for its comrade to be repaired? And even with forces combined, can they defeat the monster before all of Japan is destroyed?