In a time of prosperity on Earth, the president of the Terran League sends the starship Rugger-Guard on a mission to explore beyond the galaxy. Soon after the voyage begins, a ship of the Galbeston Empire attacks. To defend Earth, the crew deploys Dairugger XV, a super robot meant for exploration and peacekeeping, formed by combining fifteen Rugger units drawn from Land, Air and Sea assault teams. Fate then binds them to the people of Galbeston, who must find a new planet before their own explodes and must be freed from their tyrannical Emperor. The fifteen-unit design takes its cue from the number of players in a rugby union side.