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 chiropteran, and learns that she is the only one who can defeat them. (Chiroptera is the name of the order of biological classification that bats are in. This is the most likely reason for the vampiric creatures being called "Chiropterans"). Armed with her sword, Saya embarks on a journey with her family, allies and faithful servant, Haji, to rid the world of Chiropterans and rediscover her identity.

-Wiki