Rin, Rin's cousin who moved to Tokyo five years ago due to her parents' work, ends up staying at her relative Yuzuru's house for one summer week. Yuzuru is two years older than Rin and is like an older brother to her. They used to play together, catching bugs and such, when they were little. Rin reunites with Yuzuru after a long time while her parents are away working part-time, but she has grown into a cuter girl, and her breasts have grown to unbelievable heights, even though she was still a child. Yuzuru, who usually masturbates, finds Rin naked in the bath and can't control his erection.