Sosuke Sagara is an elite soldier employed by the independent mercenary special force Mithril which fights terrorism across the globe. Using battlefield technology far more advanced to that of most nations, Mithril intercedes on behalf of justice. Second Lieutenant Sagara's newest assignment is to protect a Japanese high-school girl, Kaname Chidori.

To accomplish this task, he pretends to be a new transfer student attending Kaname's school. As Sagara attempts to settle into his new role, a massive plot unfolds beginning with the kidnapping of Chidori. Why is Chidori the target of this conspiracy? Why does she seem to be born with knowledge of technology that has not yet been invented?