Hinako, the younger sister, is asked by her sickly older sister to have "baby-making sex" with her brother-in-law, who is also her husband. Hinako is confused because she feels her brother-in-law's lewd gaze on her every day, but she decides to accept the request for her sister's sake. However, contrary to the sisters' earnest desire, her brother-in-law becomes so absorbed in sex with Hina that he forgets about making a baby. Hinako's body and Kokoro also gradually change...