Yuuta Segawa is a university first year student. He and his older sister were orphaned as children but when their relatives tried to separate them and send them to live in different homes, Yuri, his sister, 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…