Yumi is a married woman living with her husband Akira in Tokyo. One day, while they were living a peaceful life, Akira's younger sister Yuuna runs away from home and comes to stay with the Kazama family. Yumi is jealous of the unusual intimacy between Akira and Yuuna and treats her harshly. When Akira finds out, the relationship between Yumi and Akira cools off, and she ends up in a forbidden relationship with Yuuna...