When she came to my house to play, my younger sister, Yuri, who came with me, was very interested in the "sex act" that I learned in class. Yuuri is like that, but only her breasts are so big that even adults are surprised by their growth. She's just insensitive to the charm of her own body and wants to experience "sex", but she innocently seduces me with her boobs... Eventually, after crossing the line, I came to visit my house regularly and was pestered for 'the act of pleasant SEX'.