Before the story proper, the viewer is given a glimpse of what happened twelve years ago, during the first heterodyne strike. Appearing from another dimension via an energy anomaly in the atmosphere, the silent but deadly enemy lays waste to Tokyo. Conventional weapons are all but ineffective; backed into a corner, the battered military drops an over-explosion mine on their foe, wiping out a large portion of the city in the process. After the atrocities of the first conflict, the mighty Dai-Guard is built to counter future heterodyne threats. But after many years of peace, the giant robot is deemed useless by the military, and is transferred to the civilian 21st Security Corporation. As the story begins, the employees of Publicity Section 2 are hawking their security products at a convention, while the brightly-painted Dai-Guard is relegated to the role of a kiddie attraction that even the kids won't take seriously. And then, the new crisis erupts. Once again, the military's best efforts prove futile, and with no combat experience, the staff of Publicity Section 2 is ordered to collect the Dai-Guard and evacuate. At this point, hot-headed Akagi Shinnosuke takes matters into his own hands; dragging two of his coworkers into the cockpit (it takes three people to pilot the Dai-Guard), Akagi enters the fray to help the attendees evacuate. Two titans clash, and the hapless office employees wage a desperate struggle: armor plates buckle, actuators burn out. But the trio of coworkers win by the skin of their teeth, earning newfound respect from the adoring public.