My cousin Rin, who moved to Tokyo 5 years ago due to her parents' work, decided to stay at her relative Yuzuru's house for a week in the summer. Yuzuru is an older brother figure, 2 years older than Rin. Yuzuru and Rin used to play together often since they were little, such as catching insects. While Yuzuru's parents were also absent working part-time, Rin, whom Yuzuru hadn't seen in a long time, had become cute and her breasts had grown unbelievably. Yuzuru, who usually only masturbates, couldn't stop his erection when he saw Rin naked in the bath.