Aoki Soubi sets out on a quest to visit his maternal grandmother's birthplace on an isolated island, since he is an orphan. Once there, he meets Tenyuu, who informs him that only males live on the island. Tenyuu already knows Aoki Soubi and kidnaps him, disclosing that Aoki is the last in a line that can bear female offspring. Tenyuu, along with his siblings Yuki and Kaitou, become Aoki's allies with the goal of impregnating him.