Kazuna comes from a family with a hereditary illness that causes an uncontrollable, vampire-like craving for blood. Unaware of this, he was sent as a child to be raised by the Eda couple. When the disease starts showing symptoms within himself, he encounters his older sister Chizuna, who has been afflicted by it since early childhood. Understanding the heavy burden this sickness places on its sufferers, Chizuna attempts to lighten her brother's load and help him manage his urges, but—