One day, the protagonist's older sister died. Upon hearing this, his mother went mad and began referring to the protagonist by his sister's name, "Natsuki." The protagonist slowly accepted this role to make his mother happy. His mother even went as far as to enroll him in the school she used to go to, Momoyuri Academy-- an all-girls school.

Momoyuri Academy is a famously prestigious school, where the daughters of company directors, politicians, bureaucrats and the like attend. Within the student body lies the descendent of the Kamikura zaibatsu, Kamikura Sayaka. A graceful young lady excelling in schoolwork and sports, Sayaka is the object of every student's admiration, including those of the so-called "Natsuki."

A few months after Natsuki transferred to Momoyuri Academy, his secret came out to his beloved Sayaka that he's a boy. Natsuki began to panic, but Sayaka promised not to tell, as long as he would do everything she said, starting with calling her "Onee-sama."

Before he knew it, Natsuki was caught up in Sayaka's orders...

[translated from official site]