A veteran in mature women, Ruby presents an incest drama. Ten years ago, his father died, and since seven years ago, Takeshi has been in an incestuous relationship with his mother, Misuzu. Even after seven years, the forbidden relationship becomes monotonous. One day, he confides in his cousin Hiroaki, who is in a similar situation, about his relationship with his mother. Hiroaki reveals, 'Actually, my family is the same way,' acknowledging their shared incestuous relationship. The two then plot to 'exchange mothers.'
