A family of five, consisting of a couple who remarried each other with their respective children, gathers for breakfast. The wife's side includes two close-in-age sisters, and the husband's side includes a college-aged son. The parents plan to travel to Atami for a three-day trip starting that afternoon. That evening, when the son returns home and relaxes, the youngest sister enters the room and aggressively approaches him... and from the next day on, the other sister also joins in, beginning a threesome life with their stepsisters.
