Saya, the protagonist, has lost her memory and lives with her adoptive family. Her daily routine appears fairly ordinary, save for the fact that she consumes far more food than is necessary. One day, she encounters a man named Hagi in a park. She learns that he has been searching for her for a long time. From that moment on, her normal life changes completely. Saya, a high school girl, cannot recall anything from her past. A handsome young man, Hagi, appears, and something about him seems familiar. This girl is more than human, possessing an ancient and mysterious past. She hunts down and kills monsters known as Chiropterans.