Second-year high school student Ito Narita moves home after her mother gets remarried. What she didn’t tell her daughter is that her new husband has four sons, all of whom are younger than Ito. Now living with Gen, Raku, Shu, and Rui, Ito is determined to be a model big sister, although the coolly distant Gen – a single day younger than her – may prove a bigger challenge to get along with than Ito anticipated.