To fulfill her older sister's wish for a child—something the sister cannot achieve due to poor health—younger sister Hina decides to allow a sexual relationship with her brother-in-law. Starting with pure intentions, Hina soon finds her brother-in-law obsessed with her young body, making increasingly intense demands. As their encounters grow more passionate, Hina gradually becomes aware of changes within her heart and body, drawn into a whirlpool of new emotions. What began as a sacrifice for her sister slowly intertwines with her own deepening feelings.