Shuji and Chise are high school seniors in a small town who have just started dating, when Shuji discovers that Chise has been engineered by the SDF so that she can transform herself into a powerful weapon. While Shuji and Chise keep trying to nurture their relationship, Chise continues to grow even more powerful as the Ultimate Weapon, and becomes increasingly torn between being a destructive fighting force and remaining an ordinary teenager. What do you do when the girl you love becomes a weapon of mass destruction?