30-year-old Aya Isaka is an amateur married woman in her eighth year of marriage and mother of five children. Her husband is a typical "Maso-san," a son-in-law who inherited her family's surveying company and now serves as its second-generation president, providing her with every comfort imaginable—home, car, vacation villa—making it a reverse dowry marriage. He is extremely strong in bed, highly enduring, making passionate love to her multiple times in a single night, which led to her bearing five children. Recently, as the children have grown capable of staying home alone, she's found herself with more personal free time. One day, invited by a high school friend, she attended a drinking party—where an unexpected encounter awaited.