Twelve years ago, Japan was attacked by a Heterodyne, a mysterious monster that seemingly appeared out of nowhere. In response, 21st century Security built a giant robot called Dai-Guard to combat the Heterodyne, but by the time it was ready, they had disappeared again. Turned over to the company's PR division, Dai-Guard has since become little more than a tourist attraction. However, when the Heterodyne reappear, PR Division worker Akagi Shunsuke is insistent that he and his co-workers should pilot the robot for the purpose it was meant for.