Gosai to musuko: Midara shiri na kusamete

後妻と息子 淫ら尻なぐさめて

Also known as 愛情と性欲　義母の息づかい, Aijô to seiyoku gibo no ikizukai

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...