At Kusunoki Girls Academy, Sayo Mochizuki (Maino Maya) lives an ordinary student life, unaware that she is actually the daughter of the head of the Mochizuki crime syndicate, secretly managed behind the scenes by her mother, school principal Yukiko. One day, a mysterious girl suddenly appears before her, claiming to be her "younger sister." This unexpected arrival will bring drastic changes to Saya's life.