One day, when Shota comes home, he finds a girl he doesn't know talking to his mother. The girl is Sachiko, the daughter of his mother's friend. She's come to Tokyo from the countryside and looks plain at first glance, but Shota is excited by Sachiko big breasts, which are visible even through her clothes. He then decides to find an opportunity to play a naughty prank on Sachiko's huge breasts.