From Viz:
Kippei Katakura is a 17-year-old playboy who spends his time chasing girls, careless of their feelings. But when his 5-year-old cousin Yuzuyu comes to live with his family after her mother's sudden disappearance, Kippei is put in charge of taking care of her. As Kippei gets to know Yuzuyu and starts to understand how she feels, he also begins to realize that all girls were like Yuzuyu once.