Itsuka Shido is a high school student. On the last day of his spring vacation, a blast devastates his town and a damsel in armor appears before him. Kotori, who is not only Shido's seductive half-sister but also the commander of the Ratatosk military organization, reveals to him that the damsel is in fact a spirit disrupting the balance of space-time. The only way to put an end to this disaster is for her to finally discover love. A task that Shido will find difficult to accomplish.