Powered by an Astro-Wave Flux Reactor, the Excel Fighter Thunder Robo was once a top-of-the-line mechanical soldier used by the warring nations of Earth. However, once the planet was united under the banner of the Terran Empire almost all Thunder Robos were dismantled and destroyed as newer, smaller defensive units became the only necessary peacekeeping forces. The only one left was kept functional, though disarmed, for ceremonial purposes and for historical curiosity, until the arrival of the Galaxians, a refugee fleet of alien ships of all different types and designs. The Galaxians were a loosely-united group of different alien species on the run from the fearsome mechanical minions of the Star Devil Emperor.