Vampires, supernatural entities who feed on the life force of the unwary at night, have existed for millennia. Karin Maaka, a high school student, is different even among her peers. Karin, unlike her vampire relatives, has suffered from polycythemia since she was a toddler, a rare illness that causes her to generate enormous amounts of blood on a regular basis. And the more blood she generates, the more anemic and lightheaded she becomes, eventually resulting in nosebleeds. Her sole option? Force her extra blood on random people, which strangely makes these "victims" brighter and happier than before. No one knew about her powers because her siblings—Anju, her reclusive though caring younger sister, and Ren, her womanizing elder brother—helped her keep them hidden by modifying the memory of the afflicted individuals.