いますぐお兄ちゃんに妹だっていいたい!

Rikuto Mitani gains a younger brother when his father remarries, except Ayumu is a girl under orders from her own mother to pass as a boy at home. The two enter Asaoka Academy together, where Ayumu attends in a girl's uniform. Rikuto's new classmates each turn out to be missing something: Matsuri has lost her sense of what being a girl means, Mao dresses as a boy, and the class representative Kimika wants to change herself. Rikuto and Ayumu start helping them.