Unstable Castle of Tokarev

Suzuhara Ayaka deeply idolized her best friend, Shiroyama Hiroko. But when they were in high school together, Hiroko dropped out and vanished, telling Ayaka only that it was for something very important. Now, a letter from Hiroko to Ayaka has washed ashore with the corpse of a girl, asking for help. Chasing after Hiroko, Ayaka goes to visit a strangely isolated high school for girls, which seems creepily eager to accept Ayaka as a student. Will Ayaka be able to uncover the school's secrets and rescue Hiroko before it is too late?