Mishiro Mikuru, a sincere and kind-hearted sister, has a caring side when it comes to her older brother. One day, after oversleeping, her brother asks her to help him change clothes, and when she enters his room, she's shocked to find his penis fully erect from morning arousal. Frozen in surprise and embarrassment, Mikuru is suddenly caught off guard when her brother, turned on by her reaction, impulsively lets her touch his cock. From that day on, he begins using his younger sister for sexual release. The story unfolds through their everyday lives, gradually depicting the shifting nature of their relationship and the emergence of a forbidden, intimate bond between brother and sister.