In Kagurahara Jogakuen, which has been around since the Taisho era, it is said that there was a secret exchange among the girls called "Pseudo Sisters". It is a special relationship in which the girls would exchange their most important possessions and sign a contract to become sisters until graduation.

However, one day, the two girls who signed a contract to become "Pseudo-sisters", committed double suicide, and the teachers treated it as an accident.

In modern-day, a series of strange incidents fall upon to five girls who attend Kaguhara Girls Academy.

One day, A transfer student named Yuuka joined the koto music club while practicing for a competition. However, while Yuuka listens to the club's performance, she said that it is an "insult to music".

Suzu gives some thought to the club president, who receives a note from Yuuka saying "I want to talk at the old school building". However, later that day they discovered the body of the president covered in blood, which had fallen from the second floor of the old school building.