Minatozaki Sana, a  spontaneous and fun-loving city-girl, transfers to a new neighborhood and high school after the remarriage of her mother. More preoccupied with boys and shopping than studying, Sana struggles to fit in at the conservative girls' school and frequently clashes with the student council—specifically Myoui Mina, the hard-working, beautiful but cold student council president. As it turns out, Mins happens to be Sana's new step-sister and Sana finds herself having to share a bedroom with a girl she absolutely can't stand. The series follows the evolution of the relationship between the two girls, with the animosity slowly lessening as the two begin to learn more about each other and confusion growing as Sana discovers that she is starting to develop romantic feelings for her new sibling.