After his father remarries, Kouta Takamura is introduced to his new stepsister, Aika, and the two immediately clash, forming a mutual dislike. However, as they begin living together, Kouta discovers that Aika has a secret life of bringing boys over to her room whenever their parents are away. To keep her mother from finding out, Aika threatens Kouta, establishing a tense and complicated relationship between the new siblings.