A young couple is very happy as they are about to have a baby; both are very happy together, eagerly welcoming their child into the world. Ai Sayama's sister sees her sister happy but does not wish her well; instead, she is always envious of her sister, longing for a gentle husband like her brother-in-law. One day, Ai accidentally sees her brother-in-law being unfaithful, as he has been deprived of sexual intimacy for a long time due to his pregnant wife. Upon witnessing this, Ai's sister wants to help her brother-in-law and is ready to take advantage of the opportunity for her own enjoyment.