Vampires—supernatural entities that consume the life essence of the unsuspecting during the night—have existed for centuries. Yet, high school student Karin Maaka is peculiar, even within her own species. Contrary to her vampire relatives, Karin has endured polycythemia since childhood: a rare condition causing her to periodically generate excessive blood. As her blood production increases, she becomes increasingly anemic and dizzy, frequently resulting in nosebleeds. Her sole remedy? Compelling random strangers to absorb her surplus blood, which unexpectedly leaves these "victims" more energetic and cheerful than before. With the assistance of her siblings—Anju, her introverted but caring younger sister, and Ren, her womanizing older brother—who alter the memories of affected humans to keep her abilities hidden, no one suspects a thing. That changes when Kenta Usui, a newly transferred classmate, finds her conduct suspicious. Complicating matters further, Karin senses her blood reacting abnormally to Kenta's presence.