教えてっ！おねてぃー

One day, Toko, Makoto's cousin, starts to live with his family because she starts to work at his high school as a practice teacher. And his parents leave them and go abroad on vacation. Makoto used to play often with Toko when they were children, but he is surprised to see her body. She's become sexy! What will happen to his life with Toko...?