The time is the Taishou Era.

The protagonist is a high class girl who stifles her true feelings for the sake of her family.
By chance she became aware of the existence of "Oni" because of a series of strange murder incidents.

One day she was attacked by a "Stray Oni" that had lost all reason and was saved by a man named Kusaka. There she became aware of the fact that she had the qualities of a "Master".

From then on her life completely changed as she enrolled into Seiryouin, a school where Oni's and their Masters make contracts and learn together.

While getting used to her new environment the protagonist learns that the strange murder incidents are the work of Stray Oni's and makes the decision to fight back together with her partner.