Yui Komori parted ways with her father, a priest, when he left Japan for Eastern Europe and a new assignment there. Since then, she has been living with distant relatives previously unknown to her. In all the time that she has spent with the Sakamaki brothers, she has learned very little about what originally caused the rifts in their relationships with one another. As she gives more of her body, heart, and soul to one of the brothers, she is told their family history.

The brothers are related through their father, Tougo Sakamaki, but the man—also known as Karlheinz—had three different wives. Shuu and Reiji are sons of Beatrice; Ayato, Kanato, and Laito are sons of Cordelia; and Subaru is the only child of Christa. The wives engaged in power struggles to gain favor with Karlheinz, and as a result, their children often suffered the physical and psychological consequences of that in-fighting.

The stories about Cordelia, in particular, strike a chord with Yui. She is uncertain why the mention of that woman affects her strangely or why her heart beats more erratically every day. Unknowingly, she carries a secret within her body that explains why her blood is so delicious to vampires as well as the nature of her familial ties to the Sakamakis.