Ichiru Kanzaki is given an offer he can’t refuse: to clear his entire debt, he has to enroll in the high school where his father works to track down his father’s illegitimate daughter, his own younger half-sister, whom he has never met. But which girl is she?

Average young girl Kazuki Suou lived a peaceful life with older brother Masaki. That is, until she met Shirou Kusanagi, a boy with secret knowledge of her past, and the dark contract Kazuki was a part of in a former life.