When four-year-old Kun's new baby sister, named Mirai (meaning 'future'), quickly wins the hearts of Kun's entire family, his world is turned upside down. As his mother returns to work, Kun becomes increasingly jealous of baby Mirai, until one day he storms off into the garden, where he encounters guests from the past and future, including his sister Mirai as a teenager. Together, Kun and teenage Mirai go on a journey through time, uncovering their family's incredible story.