The Yazaki family — four brothers raised by their father — and the Kinomoto family — five sisters raised by their mother — have lived next door for years, each quietly wishing for the child they never had. The parents come up with a plan: swap one child from each family for three months. The Yazakis' second son Kyouichi and the Kinomotos' second daughter Sakuya change homes. As the deadline approaches, both families realize the experiment has quietly become something harder to walk away from.