Life has been relatively normal for Saya Otonashi, or at least as normal as possible for someone suffering from amnesia and anemia. However, when she is attacked by a strange creature, known as a chiropteran, she suddenly regains some of her memories and realizes that she must do everything she can to track down those creatures and destroy them. With her sword by her side, she embarks on a journey to find her true identity, and with the help of the allies that she gains over the course of her journey, put an end to the chiropteran menace once and for all.