Aliens in a spaceship decide to conquer the Earth and begin by attacking the headquarters of the Getter Robo team while the Mazinger was being repaired so it could not go to their aid. One of the team members, Musashi, is killed in the battle with the alien monster Grangen and his vehicle is destroyed, meaning they can no longer form the first Getter Robo. However the laboratory already had a new Getter Robo ready for action named Getter Robo G and a new third pilot is elected. Meanwhile, Great Mazinger and his allies fight the invaders' giant robot Bong but it was just bait to lure it out so that the invaders could use their trump card: Pikadron. Getter Robo G arrives to help, but their combined beams only make Pikadron grow bigger. However Great Mazinger's pilot Tetsuya figures out how to strip this defense from the monster and destroys it with a new weapon. The alien spaceship is also destroyed and in the end everyone gets together to grieve the loss of Musashi.
