2292 AD. Earth is finally at peace, guarded by the ever-vigilant Earth Defence Force. Bored with a life of automated luxury amid the glittering towers of Artificial City 5, student Tomoru Shindo craves the adventure and excitement of the past. But while he loses himself in virtual reality dreams, a vast race of lethal machine warriors has chosen Earth as their next target for invasion. Tomoru is about to be given more adventure than he can handle.

Earth's only hope of survival is the Detonator Orgun unit, a renegade cybernetic solider being rebuilt deep within the EDF's HQ. Constructed from alien data by the beautiful Professor Kanzaki, all Orgun needs to function is a pilot, namely Tomoru.