Fukumori Daichi, who will be in his 2nd year of high school this spring, has lied that he has a younger sister in order to have a topic of conversation with a girl he has a crush on. Just when Daichi was about to tell the truth, a mysterious beautiful girl who calls him "Onii-chan" appears! Daichi is surprised to see his sister who came out of a lie, but an even more startling situation awaits him! B-B-Before something happens... he has to ask his sister!