Saya Otonashi appears to be an ordinary girl leading a mundane life with her adoptive family in Okinawa City. In reality, her only peculiarities are suffering from anemia and being unable to recall any of her life beyond the previous year. However, Saya's forgotten past quickly returns to haunt her—one night, she is attacked at school by a creature that feeds on human blood. Just when all hope seems lost, a mysterious man named Haji appears and temporarily fends off the creature. But when her rescuer forces her to drink his blood, Saya suddenly enters a trance and slays the monster with ease, using her own blood as a catalyst. Saya then learns of an organization named Red Shield, founded for the sole purpose of defeating these hellish beasts. Now Saya and Haji must work together with Red Shield to fight these fearsome monsters and unlock the secrets to the girl's past.