At 16 years old and in his second year of high school, Aizawa Haruki is a fan of an efficient lifestyle. During his summer vacation, things take an unexpected turn when Aizawa Himari shows up out of nowhere, addressing him as "Onii-chan" and insisting she's his younger sister. Upon further investigation, it's revealed that Haruki's dad recently got remarried and his new wife came with a daughter in tow. Since their parents are both overseas, Haruki and Himari are stuck living together. However, for some unexplainable reason, Himari is intent on spending every moment she can with her new older brother.