"Hoshi no Zanzou" ~Kehaizaka Jogakukan no Kinki~

The Maiden's Academy, Kewaizaka Girls' School.
Saya Karenuma transferred here to search for her "big sister," Sana Shizuki, who vanished ten years ago.
Saya slips into the school at night, all the while chasing answers surrounded by classmates tormented by erotic nightmares.
Everyone—students and teachers alike—sees echoes of Sana in Saya and acts weird around her.
It's like everyone's reliving some old scandal; one after another, the girls start to lose it.
Can Saya uncover the secret behind that disappearance? When morning comes, will she finally grasp the truth?