A remarried couple and their respective children from previous relationships—a family of five consisting of two sisters close in age from the wife's side and a college-aged son from the husband's side—are having breakfast together. The parents are going on a three-day trip to Atami starting from noon. That evening, as the son is relaxing at home, his youngest stepsister enters his room and starts pushing him... And from the next day, the other sister joins in, beginning their threesome life with their step-sisters...
