Karin is the oldest daughter in a family of Japan-based vampires. However, Karin is different from her parents and siblings. Karin doesn't dine on the blood of others. Instead, once a month, her blood supply increases on its own. Then Karin meets Usui, the new guy transfer student. Karin's crush on the new boy causes her blood volume to surge to an almost overflowing volume! Even worse, Usui discovers Karin's secret...