During her spring break before starting her first year of high school, Ito Narita (formerly Ito Hasui) moves to a new town after her mother remarries. Expecting a quiet new life with just her mother and new stepfather, she is shocked to discover that her new family also includes four handsome stepbrothers. Suddenly becoming the older sister to four boys, Ito resolves to do her best to bring her new family together.

The four brothers each have distinct personalities. The oldest, Gen, is a high school sophomore just one day younger than Ito. He is brusque and struggles to express his feelings, having shouldered the burden of supporting his brothers since their mother passed away. The second brother, Raku, is a calm and intelligent high school freshman who often acts as a strategist, smoothly maintaining the balance within the family. The third brother, Shu, is a mysterious and shy middle school student who tends to isolate himself in his room. The youngest, Rui, is an elementary school student who is cheerful, affectionate, and the family's adorable mascot.

As Ito diligently works to get along with her new brothers, she faces unexpected challenges. Gen is cold and distant toward her, leading Ito to feel that she is unwanted in the Narita household. However, she gradually discovers that Gen's harsh demeanor masks a deep devotion to his family. The initial conflict revolves around Ito trying to find her place and Gen learning to accept her help. A notable narrative arc involves the brothers, especially the reclusive Shu, developing unique feelings for Ito, which adds a layer of romantic complication to their new sibling dynamic. The story follows the Narita family as they navigate the chaos and heartwarming moments of living together under one roof, slowly growing from strangers into a true family.