A family of five, consisting of a couple who have remarried and have three children (the wife's sister who is close in age, and the husband's college-aged son), sits down to breakfast. The parents decide to go on a three-day trip to Atami in the afternoon. That evening, when the son returns home and is relaxing, the youngest sister comes into the room and starts to make advances on him...Then the next day, the other sister joins in, and so begins a threesome with the step-sisters...