The couple Reo and her husband live next to the room where Mika, Reo's younger sister, and her husband Shun have just moved in. One day, Mika sees a man being brought into Reo's place and consults with Reo's husband Yuichiro. However, Yuichiro remains calm after hearing the story, and Mika is embraced and toyed with by him. Later, from some of her attitudes, Reo finds out about Mika and Yuichiro's relationship, so in retaliation, Reo starts to seduce Shun, and before long, the sisters and their husbands begin to swap partners…
