Rei, a married woman living happily with her daughter and son-in-law, discovers her missing underwear and catches her son-in-law Takehito masturbating with it. She learns that the couple hasn't had sex in a while, and Takehito is frustrated that his huge cock remains unused. Feeling sympathy for his loneliness, Rei becomes aroused and seduces him, saying, "Let me help you in my daughter's place." Despite knowing she's his mother-in-law, Takehito is turned on by her aggressive behavior and proudly reveals his rock-hard, massive cock, beginning a new forbidden relationship.