The concept of normality is not equal for all people. It is relative and depends on many factors, such as their country of origin, economic condition, people around us and the lessons they teach us. To Sousuke Sagara, a member of the military organization named Mithril, normality is to move from one battlefield to another, always ready to stake his own safety in the name of higher ideals like peace, justice and freedom. For a guy like him, life led by ordinary civilians is nothing more than something to observe from a distance with a certain detachment. But his routine is disrupted when his superiors assign him the task of safeguarding the safety of Kaname Chidori, a seemingly normal high school student, but that actually has some knowledge (unknown even to her) that could constitute a dire threat to the whole world.