Work forces Hiroshi to stay with his older brother and beautiful, voluptuous sister-in-law, who looks remarkably youthful for her age. The adult virgin gradually becomes obsessed with living under the same roof as her. Secretly watching her masturbate, bathe, and have sex with her husband soon becomes his favorite excuse to jerk off. Then one night his brother asks, “Isn't it time you stopped peeping?” Hiroshi realizes the couple knew everything all along.