Mirai tells the story of Kun, the first child of a young married couple who lives in Japan.
Kun gets all the love from his parents until one day, they return home with their newborn daughter. Not getting the same attention as he used to, Kun becomes jealous with her sister and starts doing naughty things just to get his parents to notice him. However, they are so busy with work and the new baby that Kun feels like he was abandoned. Until one day, he finds his dog turned into a grown-up man and is greeted by none other than his sister, Mirai, who comes from the future and is now a teenage girl. From that day on, Kun experiences different events that happen both in the future and the past and are some how related to his family from generation to generation.
Kun is the main character of the movie and director Mamoru Hosoda has shown that he understands children really well through the emotions and the mind, specifically the imagination of Kun. The movie is simply the story of a family's daily life that is told through the eyes of a 4 years old child and the process of how that child can become a loving son and a loving brother. The narrative of the movie is just like the way Mirai Mirai (Mirai is the name of his sister which means future in Japanese) told his brother about how their family came to be through those little things that happen every day in our lives.
Mirai is a film that shows you how wonderful a child can be, and although Kun is very jealous with his newborn sister, deep down inside he loves her very much.