When he met his younger sister Himari for the first time in a long time, she seemed completely boyish and unattractive. One day, a month after returning home, his parents were working the night shift and wouldn't be home until morning. Toshiya, who was left in charge of all the housework, couldn't help but feel his urges when he saw his sister's defenseless appearance. Seeking her breasts, which were transparent with sweat, he played with her nipples and sucked on them. The younger sister also rubbed her nipples against her brother's penis and started tweaking them. They moved to the bathroom, where they still desired each other...