Saki is concerned about her younger brother Mamoru, who has not been feeling well lately. Apparently, Mamoru, who does not yet understand the meaning of sex, is being bullied at school. Saki answers the questions of the dejected Mamoru. Saki tells him that "siblings cannot have sex," but after an incident that Mamoru causes at school, Saki decides to step in and do something for her cute little brother.