Overview:

Summer vacation. The main character, Takuma, ends up living with his younger sister, Hiyori, while his parents go abroad.
Hiyori used to be honest and cute, but lately she has become quite brash.
Takuma wishes for Hiyori to return to her old self, but Hiyori’s impertinent behavior only escalates…
Takuma finally reaches the limit of his patience and decides to train Hiyori, a brash female brat, to make her understand and return her to an honest Hiyori.