The series is based around three office workers of the 21st Century Defense Security Corporation's Public Relations Division 2 who, with the company's giant robot, Dai-Guard, fight interdimensional alien beings called "Heterodynes".
Unlike most giant robot anime, Dai-Guard performs on much more realistic physics, making it heavy, slow, and difficult to control.
Also, the Heterodynes seem to be the least of the 21st Century Corporation's problems as they experience opposition in the form of massive damage claims, lawsuits, rivals and a jealous and antagonistic military who wishes to take back Dai-Guard for themselves.
However, no further attacks occur for the next twelve years and the 21st Century Corporation is allowed to keep the useless weapon as a mascot which is managed by Public Relations Division 2.
However, during a security exposition in 2030, a Heterodyne attacks and the ill-prepared and unarmed robot is taken into battle by its pilot Akagi Shunsuke.
His headstrong nature, combined with the talents of his fellow crew, enable Dai-Guard to be victorious against multiple Heterodyne attacks causing the military to want it back.
Its first battle was an outstanding success, but in the second one the Heterodyne fused with the robot with the pilot still inside who was rescued by Akagi in Dai-Guard.
From episodes 2-25, the opening theme is "Rojiura no Uchū Shōnen" (路地裏の宇宙少年, "Back Alley Space Boy") by The Cobratwisters (ザ・コブラツイスターズ, Za Koburatsuizutāzu) while the first ending theme from episodes 1-25 is "Hashire Hashire" (走れ走れ, "Run, Run") by Kyoko Endou (遠藤響子, Endō Kyōko).