When Shiho Nishina got worried about her sex life with her husband, her husband proposed to her, "Why don't we go to a couple exchange party?" At first Shiho strongly turned down the idea, but eventually, she gave in because her husband was very persuasive. "We're just going to observe, okay?" Shiho said as she reluctantly agreed to it. Meanwhile, a former diplomat Mr. Kubozuka ran a couple exchange club, "Aquarium," with his wife in their house. At first, Shiho was being very cautious, but gradually, she got attracted to Mr. and Mrs. Kubozuka's charm and finally agreed to exchange couples with them. It seemed that their sex problem had disappeard and they had found happiness, but as it turned out, that was just the beginning of their nightmare...