Variety describes the film as such:
Mirai follows a 4-year old boy who is struggling to cope with the arrival of a little sister in the family, until things turn magical. A mysterious garden in the backyard of the boy’s home becomes a gateway allowing the child to travel back in time and encounter his mother as a little girl and his great-grandfather as a young man. These fantasy-filled adventures allow the child to change his perspective and help him become the big brother he was meant to be.
In an interview with Variety, Hosoda mentioned that his films always have a common thread.
There is a common thread in the themes of my films: The Girl Who Leapt Through Time was about youth, Summer Wars was about family, Wolf Children, Ame and Yuki were about motherhood. The Boy and The Beast was about the father, and my new film is about the relationship between brothers and sisters. Mirai is about a boy who is trying to reclaim the love of his parents. I always strive to make films that have a universal dimension in the choice of subjects and even the dialogue.
Mamoru Hosoda is definitely one of the biggest deals in Japan and around the globe as well, as he’s often cited to be the successor to Hayao Miyazaki and Studio Ghibli.
The movie is expected to be completed in May 2018.