Due to the attack from the unidentified life forms, Tousuien City was flooded with blood of the victims. The tragedy is called Tousuien Incident, and among the survivors is a student named Senba Shinkurou. Shinkurou lost both his parents from the incidents, so he carried the loss with him when he grew up alone. One day, Shinkurou encountered the same monster from Tousuien Incident. Facing each other, Shinkurou was outmatched and fell into a dangerous situation. In that moment, a beautiful female knight appeared and saved him.

Her name is Asklein Christa. She is a transformed heroine of justice who came to Shinkurou's world to hunt down the monster... the Fallen Demon. Christa's real identity is Luana, who actually a beautiful member of a royal family from the other world. Since she came only to exterminate the Fallen Demon, she has trouble living in Shinkurou's world. So, she moved in with Shinkurou to adapt to the world and help him out. During the fight against the Fallen Demon, Shinkurou and Luana deepened their bond together.

In the meantime, Sayaka, who is Shinkurou's childhood friend, just finished her miko training at a Shinto shrine and was returning. When Shinkurou found out that she fought against the Fallen Demon under the alias of Sukuna no Himemiko, the relationships between the three of them became more complicated.

[Translated from Getchu]