Tounomori Iota had a certain problem during his summer vacation. It concerns Teruru-chan, who recently started staying at his house. She is an outrageous magical girl with no noble purpose or goal in mind, and she does all sorts of naughty things whenever and wherever she goes. Even his older sister Rin, who was initially just watching over them, can't help but join in, and every day the Tounomori family is in a state of sexual mayhem!