Goro is a high school student who just lost his mother but has older stepsister Haru. She likes to play naughty tricks and games on Goro everyday, which makes him troubled and annoyed. It is very hard for Goro who is still virgin. Goro`s classmate Mai feels suspicions about unusual relationship between Goro and Haru. And gradually Haru’s tricks are escalating and going beyond funny jokes.