Kun is a spoiled little boy. But ever since the arrival of his new baby sister Mirai, Kun misses his parents’ affection and is baffled by things he has never experienced before. Then one day, he encounters a mysterious girl named Mirai, who calls him brother. A magical journey ensues as Mirai and Kun travel through time where he meets a mysterious man who claims to be a former prince, his mother as a child and his father as a young man. Through his adventures, Kun learns life lessons and how to cope with living with a baby sibling. How will his journey end? And why did Mirai travel back in time?