Living with her adoptive family in Okinawa City, Saya Otonashi leads an unremarkable life. The only complication is her anemia and the fact that she has lost all her memories before the previous year. However, an encounter with a blood-sucking creature at school triggers her memory and a mysterious stranger named Haji rescues her. As she delves into her past, Saya discovers the existence of Red Shield, an organization committed to eliminating these demonic monsters. With Haji by her side, she joins Red Shield's battle to uncover the truth behind her past and confront the monstrous creatures.