Iori is a high school student. He has a girlfriend, Miki, but she recently gives him a cold shoulder. One day, Miki says, "Why don't we try swapping sex?" Iori can't understand what she means at first. But he doesn't want to be dumped so he accepts it. And she chooses Yuki and Shoko as swap partners. Iori is surprised to know that because Shoko is the girl Iori had his first sex with. Iori is totally confused....