Eight years ago the city of Tonsuien was stained with blood due to an attack by a group of strange creatures. The student, Makuro Izumiha is a survivor of the terrible incident known as the Tonsui-en Incident. Ever since he lost his parents in the incident, he has been living alone with old wounds in his heart. One day, Makuro witnesses the same deformed monster from the Tosuien incident. When he confronts the monster, he finds himself in a critical situation in front of the overwhelming power of his opponent, but a beautiful female knight saves him from his predicament.

Her name is Asclane Crysta. She is a righteous, transforming heroine who has come from another world to hunt down a deformed monster the Fallen Demon. Christa's true identity is that of an otherworldly royalty, a beautiful woman named Luana. Although she came from another world to exterminate the descending demon species Luana was having trouble living in this world, so she came to live with Makuro, who was living alone for a while. Luana and Makuro deepen their bond while overcoming the battle against the descending demon species.

Then Sayaka a childhood friend and a winsome girl who works as a shrine maiden at a nearby shrine, returns from her training. By chance, they learn that Sayaka is Sukuna no himemiko, who has been fighting against the demon species and the relationship between the three becomes complicated.