Natsu, a teenage girl more obsessed with sex than studying, spends her days locked in her room masturbating or inviting her boyfriend over for intimate encounters—all of which her third-year ronin brother secretly watches. One day, the brother sneaks into Natsu's room and fails to maintain their usual routine. From that moment on, their power dynamic shifts completely: the brother becomes her submissive servant, treated like a pet dog by his younger sister on a daily basis. Though blood is undeniable, for this brother, his sister is truly one-of-a-kind.