"Big Sister! What are you doing?" While his wife was visiting her hometown for childbirth, Toshiya was enjoying a brief taste of bachelor life. However, he was suddenly called out by his sister-in-law, Erika, from behind while masturbating. It seemed she had come to check on him, worried about his solitude. By the time he hastily tried to hide his crotch, it was too late. Seeing Toshiya's erect penis, which had been pent up due to his wife's absence, Erika approached with a knowing smile, as if plotting something. There was only one week left until his wife's due date. A dangerous cohabitation with his erotic and free-spirited sister-in-law had begun.