Yuuta Segawa is in his freshman year of college. He and his older sister Yuri were orphaned as children but when some of their relatives tried to separate them and send them to live in different foster homes, Yuri chose to raise him on her own. Now, many years later, Yuri, who has since married, asks Yuuta to babysit her daughter, Hina, and two step-daughters, Sora and Miu Takanashi, while she and her husband go on a trip. Unfortunately, the plane Yuri and her husband are on crashes and they are both reported missing and presumed dead. A scene similar to the one Yuuta experienced as a child unfolds and in order to prevent the girls from being separated from each other, Yuuta takes it upon himself to look after the girls in his small apartment. Thus begins Yuuta's story of sharing his everyday life with his three nieces.