Under the care of her adoptive family, Saya Otonashi has been living the life of an anemic and amnesiac, but otherwise ordinary schoolgirl. Saya’s happy life is shattered when she is attacked by a chiroptera, a hematophagous bat-like creature that lives by feeding on human blood, learning that she is the only one who can defeat them. Armed with her katana, Saya embarks on a journey with her family, allies, and her chevalier, Haji, to rid the world of chiropterans and rediscover her identity.

Chevaliers are the second highest level of chiropterans in terms of strength and ability. They are created when a human drinks the blood of a Queen. A powerful bond exists between each chevalier and the Queen that sired them, compelling the chevaliers to serve and protect her at all costs. Chevaliers have even great speed, strength and endurance. Though they retain the appearance in body form and age that they had when they were transformed, chevaliers have vast shapeshifting abilities.