Kazuna's family has a history of suffering from a disease that creates an uncontrollable, vampire-like thirst for blood. Unaware of this, Kazuna was sent to live with the Eda couple as a child. As signs of the sickness begin to appear in him, he comes into contact with his older sister, Chizuna, who has suffered from it since early childhood. Knowing that the illness places a heavy burden on those who suffer from it, Chizuna tries to ease her brother's burden and help him learn to control his urges, but...