Sakai and Shinohara are a couple in a relationship. One day, Sakai visits her house and meets her mother, Aiyu, who emphasizes her large breasts, causing him to become excited. While borrowing the toilet at her house, Sakai witnesses Aiyu masturbating while pretending to be Shinohara and calling his name. Unable to contain his excitement, Sakai also starts masturbating, but Aiyu reaches orgasm first, leaving him with blue balls. After that, Sakai can't get the incident out of his head and starts pursuing a relationship with Aiyu...
