Orphaned as a young child and shuttled between indifferent relatives, Mito Yamamoto, a girl disguising herself as a boy for Reasons (?), is now a teenager with no family, no home, and no job. When she is saved one night by Ruka Saotome, a vampire in need of a thrall, Mito readily agrees and begins her new life living with him in Hijirigaoka Boys’ Academy dormitory. However, when she discovers that Ruka cannot drink from girls and was therefore seeking a male companion, Mito seemingly has no choice but to keep her true identity a secret.