Kana Takachiho is a kind and helpful student, always willing to lend a hand. While heading to her part-time job, she spots her childhood friend, Aki Kirito, but mistakes him for his twin brother. As she tries to catch up to him, she sees a little girl standing in the road, about to be hit by a car. Kana rushes into the oncoming traffic to save her, only to end up nearly dead in the street. Aki, seeing the accident, cradles the dying Kana in his arms and decides to save her—by biting her!

When Kana awakens, she discovers that Aki is a pureblood vampire and she is now his "thrall," a being that will never die or age. But now her blood, her body, and her life belong to him alone. As Kana and Aki begin living together, she learns that he seeks the seven "Stigma"—magical tattoos that come with great power. When he collects all seven, he will use their power to awaken his twin brother Eriya, Kana's beloved.