I visit my wife’s family home with my pregnant wife. One day, obscene moans drift from the tatami room, and when I secretly peek through the sliding door, I find my mother-in-law Mariko masturbating. Wildly aroused, I hide nearby and masturbate along with her. Mariko notices and whispers, “Don’t tell my daughter, okay?” That night, while the three of us sleep side by side, she makes her move—and our secret affair leads to repeated sex and creampies over the three-day visit.