A family of five, consisting of a couple who remarried each other bringing their children, gathers for breakfast. The children are two sisters close in age from the wife's side and a college-aged son from the husband's side. The parents plan to travel to Atami for three days starting from noon. That evening, when the son returns home and relaxes, the youngest sister enters the room and aggressively approaches him... Then, starting the next day, the other sister joins in, and a threesome life with the step-sisters begins...
