Gosai to musuko: Midara shiri na kusamete

Original title: 後妻と息子 淫ら尻なぐさめて

Takuya lost his mother at a young age, and his father, Kazuhiko, remarried a woman named Junko—who was significantly younger—when Takuya was in high school. Kazuhiko passed away from illness four years ago, and Takuya had been living alone with Junko ever since. One day, Takuya brought his fiancée, Misaki, home to introduce her to Junko...