Shiho and Rei have always been close sisters. However, after Rei's divorce, she ends up having an affair with Shiho's son. The relationship is a secret from Shiho, but when Shiho finds out, Rei flirts with her nephew in front of Shiho as if she has given up. Seeing this, Shiho also has an affair with Rei's son. After the sisters find out about their relationships, the four of them go into the bath together, and then...