Yuzu lost her parents and was living with her sister and brother-in-law as a couple. One day, her sister asked her, "Due to my physical condition, I can't raise children easily. Please bear my husband's child for me." After much hesitation, Yuzu decides to fulfill her sister's wish by engaging in a relationship with her brother-in-law. However, he becomes obsessed with having sex with Yuzu, deepening their intimacy. As their relationship grows, Yuzu's heart gradually changes, and new emotions begin to stir within her.