Transfer student Mamoru Amakawa unexpectedly becomes the object of affection for Sakura Sumiyoi and Ageha Kurosaki, the powerful daughters of the city's rival ruling families. While the two "princesses" clash violently over who gets to claim him, Sakura takes a bold first step by seducing Mamoru in the school's nurse's office, hoping to secure his loyalty through an intimate encounter.
Mamoru and Sakura's romantic date is interrupted when Ageha kidnaps Mamoru and lures him into a secret room. Using a love potion to lower his inhibitions, Ageha seduces him and establishes a pseudo-sibling bond, insisting on moving into his home. Sakura is left devastated as she realizes her competition for Mamoru's affection has grown, especially with the arrival of his actual little sister, Kuu.