Gibo no shinshitsu: Injuku no yoromeki

Original title: 義母の寝室 淫熟のよろめき

Yuya, a university student, grew up in a wealthy lawyer's family, living a sheltered life free from want. Now, the woman he desperately desires is his stepmother, Reiko. One day, Yuya spots Reiko entering a luxury hotel with another man. He uses this discovery to blackmail her, successfully forcing her into a sexual relationship. However, Reiko counters with a condition of her own: as an act of revenge against the man who once abandoned her, Yuya must assault a woman named Yuriko—the man's current wife.