A sudden mutation strikes a certain town. A series of bizarre crimes begin to occur. Then, a girl named Ai suddenly appeared. It had never been there before, but one day it suddenly began to exist as if it were a given, something that was always there. While everyone around her paid no attention, the protagonist was the only one who noticed something was wrong with her.

Her true identity was that of a warrior sent from another world. Prioritizing the stability of the other world, "love" is not necessarily on humanity's side. What fate awaits the protagonist as he becomes embroiled in a battle against demons—"fluctuations"?