Tokarev no Ayaui Shiro  

Unstable Castle of Tokarev, トカレフの危うい城

Ayaka admired her friend, Shiroyama Hiroko, greatly in high school. However, after graduation, Hiroko disappeared without explanation, leaving only a cryptic message for Ayaka. Recently, a letter from Hiroko was found with the body of an unknown girl, requesting assistance. Driven to find her friend, Ayaka visits a secluded girls' school that seems eager to enroll her as a student. Can Ayaka uncover the secrets of this mysterious institution and rescue Hiroko before it's too late?