Shuntarou is an average high school porn addict. His sexual desires are directed toward his younger sisters, who he deeply adores in a way no older brother should. He even resorts to stealing their underwear and satisfying himself to it, for which they deeply despise him. His only wish is to have a caring and kind older sister who would be the complete opposite of his mean siblings.
After one such session with young women's panties, Shuntarou spots a mysterious new video game on his computer. Although he has no recollection of ever downloading it, the title "Older Sister Week-Long Experience" sounds too alluring to pass on. While the game seemingly does not start, the next day he finds himself shrunken to the size of an 11-year-old. When his sisters discover him, he introduces himself as Shuntarou's internet friend Koharu—and the girls instantly fall head over heels for his adorable and innocent appearance. Koharu quickly takes advantage of the situation and subjects himself to the tender care of his oblivious, now-older sisters.