Kotone is a sweet, innocent girl whose perverted older brother constantly sneaks into her room to spy on her masturbating or fooling around with her boyfriend. One night, he breaks in again with the intention of having sex with her but gets caught. From that moment on, the brother becomes his sister's sex slave, forced to perform cunnilingus on her every single day. Though they're related by blood, the mere existence of this sister completely transforms her brother's life forever.