A long forgotten myth holds truth to current day. Legend tells of alien creatures invading primitive Earth millions of years ago. These creatures, known now as Solonus, were destructive beasts bent on overpopulating Earth with their species. Leading them were super intelligent human-like Solonus who controlled their monster like counterpart. Humanity was on the brink of becoming extinct, until one man somehow obtained material and technology from the Solonus' "Home". Using the material, he crafted weapons and armor that gave humanity a chance for survival. Setsuna lives in a world where humans live unknowingly alongside Solonus, blind to their existence and the story of how they arrived to Earth. She knows of the Solonus, however, because she is one. A painful childhood and hate for the man who killed her parents lead her to resent her own kind. Thus, she creates a bounty hunting orginization who specializes in beridding the world of Solonus - "Ace Solonus"