A young girl named Saya lives with her family but doesn't know that she is not biologically related to them. She cannot even recall her past. However after being forced by a mysterious man to drink the blood of a species of creatures named Mouse, she turns into her past form, with high fighting abilities. After fighting the Mouse, Saya loses the memory of that fight and her "brother" Kai witnessing the action doesn't know what is actually going on. Throughout the anime, she slowly learns about her past but the mysterious man asks her if she's ready for the path that she will have to take.