Kun Oota, four, has had a privileged life as an only child with his parents and the family dog, Yukko, in a secluded area of the city. But when his new baby sister Mirai arrives, his uncomplicated existence is turned upside down; it's no longer all about him. Despite his tantrums and nagging, Mirai appears to be the object of his parents' undivided attention. Kun is taken on an astonishing voyage through time to help him adjust to this tremendous shift, meeting his family's past, present, and future selves as he learns not just what it means to be a part of a family, but also what it means to be an elder brother.