Akito Tatsugachi big sister, Himari, is someone he really loves alot. But after becoming a highschool student, he decides distance himself from her. This sudden change in her little brother's behaviour causes poor Himari to fall into sadness and a gloomy mess.

Seeing how much he has hurt his beloved Nee-chan, Akito decided to not only become the Otouto the loves being spoiled by his loving Nee-chan, but also an Otouto that spoils his cute Nee-chan as well.