Somewhere the Pacific Ocean, a prehistoric monster awakens after resting within the depths of the ocean for millions of years. The monster is given the name, the Dragonsaurus and it heads for Tokyo. Word is quickly spread to Japan and three Super Robots: Grendizer from the Space Science Laboratory, Getter Robo G from the Saotome Institute, and Great Mazinger from the Photon Power Laboratory; are sent to confront it. Flying at the monstrouscity, the robots encounter what appears to be a serpent. Believing this to be the Dragonsaurus; they open fire. After a short celebration, the head regenerates and more heads show up. As the battle rages on and the Dragonsaurus approaches dry land, the serpents are revealed to be merely just limbs for the real head in the center of the body. Boss Borot appears to try and get some of the glory, only to be swallowed by the Dragonsaurus. The oil within the Borot causes the Dragonsaurus to grow bigger and stronger; making the battle more difficult for the three mecha. Great Mazinger manages to wound the Dragonsaurus which causes Boss Borot to fall out of its body. Brushing the robots off, the monster approaches Tokyo and goes on a destructive rampage. Hayato Jin suggests to Ryoma Nagare to use the Getter G's Shine Spark to ignite the oil in the Dragonsaurus' body. Ryoma does so and the Dragonsaurus explodes. Danger is once again averted and the city is saved.