Second year high school student Aizawa Haruki tries to live his life efficiently. One day during summer vacation, a girl suddenly appears, claiming to be his little sister. It turns out that his father recently remarried, and his new wife has a daughter, Himari. Because both parents are overseas, Haruki and Himari will be living together. And for some reason, Himari seems very insistent on spending as much time as possible with Haruki as she can.