Keita is a Japanese schoolboy in his third year of high school who is preparing to take his high school exams. He has two half-sisters who are not related to him by blood – the beautiful Ako and Riko, who have feelings for him that are far from sisterly and show it every chance they get, whether it’s to pry into his bed or bring him lunch at recess. And then there are classmates, and pouring oil on the fire. And what is the poor guy to do?