Living a seemingly ordinary life in Okinawa City with her adoptive family, Saya Otonashi faces the challenge of persistent amnesia and anemia. However, her quiet existence shatters when she is ambushed by a bloodthirsty creature at school. Saved by the enigmatic Haji, who compels her to drink his blood, Saya unleashes dormant powers to slay the beast. Embracing her new role alongside Haji and the organization Red Shield, Saya delves into a world of terrifying adversaries, determined to unearth the mysteries of her forgotten history.