The story's about a straight-edged, boring, careful family man Makoto (i.e. "M") and Saya (i.e. "S") the daughter of a classmate whose life he unintentionally ruined. S is basically out to extract revenge for her mother by blackmailing, stalking and just making life miserable for M.
Borderline at most. Better to be safe in fapland then in weebland.