Sister and brother, Arisu and her older brother, go on a hot spring trip together. Arisu has made up her mind to confess her feelings to him, but she struggles to find the words. Instead, she awkwardly tries to express her emotions through physical actions—washing her brother's body at the hot spring, teasingly flashing her still-developing small breasts. Though her heart remains pure, her body grows increasingly erotic. Her innocent yet sensual behavior causes her brother to become erect repeatedly. That night, their relationship finally crosses the forbidden line…