Saya Otonashi is a seemingly ordinary high‑school student who suffers from amnesia, unable to recall the events of the past year. One day a man appears and hands her a katana, and her destiny begins to unfold. She soon discovers that she must confront a new threat to humanity: the Chiropteran monsters, ravenous, immortal beings that can alter their appearance and disguise themselves as humans. These creatures feed on blood and conceal themselves within the human world. The Red Shield, an organization devoted to eradicating the monsters, has been waging a clandestine war against them, and the struggle has intensified. Saya’s quest for the truth has just begun.