Shion Aoki, the Vice Captain of the Onmyou-ryou Second Unit, and Yuusei Toudou, a fellow soldier. Known as the "Twin Walls of the Empire," the two protect the nation from spirits (Ayakashi). They are childhood friends who are deeply in love and were once betrothed to each other. However, due to an injury sustained in his youth, Yuusei has been treated as a "defective product" by his family. Heartlessly, his position as the fiancé of his beloved Shion is stolen by his younger sister, Suzune. Though Yuusei harbors complex feelings, the day of the betrothal ceremony between Shion and Suzune is drawing near—.