Mikoto Kitajima lives a peaceful life with her younger brother Masaya Kitajima, even though she gets teased by her friends for being a brocon. But her life get turned over with the appearance of Kinuko Sakagami.

"You are the ideal younger brother, I want you to become my younger brother"

Kinuko moves into the Kitajima house, taking advantage of the fact that their parents are travelling abroad. Moreover, she uses her financial power to expand her room right next to Masaya's. Mikoto couldn't just sit silent in this situation. And so, begins the fierce battle for the younger brother.

"Huh...wait, both of you!?"

What will be the fate of the younger brother caught between the two older sisters?