Two giant robots that hate each other's guts have to stop bickering and actually work as a team before an alien monster tears them both apart. The movie doesn't waste time on reconciliation speeches, it just puts them in a position where their rivalry becomes a liability they can't afford. When they finally synchronize, it feels earned because the show spent the entire runtime proving they had good reasons to fight in the first place.

A space monster named Gilgilgan arrives from the far reaches of the universe and attacks Earth. A strange, seemingly living UFO appears over Japan and drops the metal-eating creature. The Science Fortress Laboratory and the Saotome Institute both move to stop the threat, but the rivalry between Great Mazinger and Getter Robo gets in the way. Getter Robo reaches the UFO first and fails to bring it down, and Great Mazinger takes a heavy beating from Gilgilgan. As the monster grows stronger and begins to change, the two bases decide to join forces, over the protests of their pilots. The rivals then launch a combined attack in this action-packed Go Nagai mecha crossover movie.