When his mother, Mariko, asks him to fetch soap while she is taking a bath, Satoshi, her son, is captivated by her enormous breasts. His mother's body, which he has not seen since he was a child, is more stimulating than he had imagined, and his groin reacts involuntarily, causing Mariko to chase him out of the dressing room. However, Satoshi cannot forget Mariko's naked body, and he masturbates while fantasizing about his seductive mother stroking him...