Mizuki, the student council president, shows a resolute attitude even towards the sexually harassing teacher, Tanabe. While feeling a sense of perversion at her beloved younger sister Aika's uncharacteristic remarks, she sees her off to her extracurricular activities… However, Tanabe calls Mizuki and shows him lewd videos of her sister. In exchange for promising to dispose of the videos, Mizuki accepts the man's demands. Experiencing something for the first time, her consciousness gradually melts away, and she trembles with pleasure…!?