Shinnyuusha

"—But still, what was that man all about...?"
A memory suddenly crossed Reiko's mind. An unknown man she had seen on an empty train, and when she got off the train, Reiko was curious and looked towards the seat where the man had been sitting, but the man had already disappeared. As Reiko washes herself alone in the shower room at home, she is approached by someone... And in her terror, Reiko awakens to lustful desires and descends into a world of depravity.

[Translated from Getchu]