Mika moves in next door to her sister Ryo and Ryo's husband Yuichiro, accompanied by her own husband Shun. One day, Mika witnesses Ryo secretly inviting another man into her home and confides in Yuichiro. Instead of being concerned, Yuichiro becomes aroused, pulling Mika close and teasing her provocatively. When Ryo discovers the encounter through her husband's behavior, she retaliates by seducing Shun. Soon, the two couples begin swapping partners, leading the story into an intense and passionate new direction.