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.

Five thousand metres above ground, Dai-Guard and Kokubogar must brave the perils of the Heterodyne's body in search of the original fractal knot. But even as they battle their way through the numerous obstacles in front of them, the clock is ticking- in less than two hours, the army are going to launch a destructive OE that will take down not only the Heterodyne, but vaporise all of Tokyo as well! Will the two giant robots be able to pull off a victory in time?