After quitting baseball because of a certain incident, Tetsuji has been spending his high school days listlessly until one day, his married elder sister suddenly returned home. She's 188cm tall and he's too scared to ask of her weight. Her name's Mao and her nickname is Maou a.k.a Demon King. Tetsu is also big-sized but he is timid unlike his sister who is huge in many ways, and they are about to spend an unexpectedly sad yet fiery summer holiday together.