Yuichi, the older brother of his younger brother's girlfriend Mai, realizes at first sight that she is a masochist who likes deep throating, and plans to train her. He stares at Mai and treats her with the utmost kindness. One day, Mai spills tea on Yuichi and gets her trousers dirty, and Yuichi makes her apologize desperately. Yuichi makes her wipe his crotch in a mischievous commanding tone. On another day, Mai comes back to apologize to Yuichi again, and he commands her in a mischievous tone to "gently stroke my penis."