An Italian theatrical film edited from Mazinger Z vs. Devilman, UFO Robot Grendizer vs. Great Mazinger, and Great Mazinger vs. Getter Robo.

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.

Mazinger Z vs. Devilman is a 1973 animated movie that crossed over two then-popular Anime series, both of which were created by Manga artist Go Nagai. While fighting with some of his enemies Mazinger Z inadvertently knocks them into a volcano, where they crash into the prison of the Satan race and release Phoenix Jenny! Dr. Hell, learning of this, frees the rest of the Satan race and teams up with them to defeat Mazinger Z, and their ability to fly proves to be too much for our hero. Meanwhile, Devilman is also individually fighting the members of the Satan race, but their combined power is too much for him as well; can Mazinger and Devilman put aside their differences and team up to defeat Dr. Hell and the Satan race?

Kouji and his friends have defeated Dr. Hell and are now enjoying a break, but suddenly a strange prophet appears and warns everyone of an oncoming danger, mechanical beasts never seen before start appearing all around the world wrecking havoc. Its up to Kouji and his Mazinger Z to stand up to this threat but it seems he is vastly outnumber and outmatched.

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 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?

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?

In the far reaches of space a new star comes into being, bringing a greedy and ambitious bionic human, Alfred, whose first port of call in his quest to conquer the universe is Antarichi, a planet of peace and perfection. Kanaan, King of the city of Orion, seeks the help of Earth, who gather together a mighty force of robot warriors, the Protectors of the Universe, to put a stop to Alfred's plans for galactic destruction.