When Leslie suddenly dies, Ben is forced to take his sheltered children into "the real world" for the first time. This transition highlights several central themes:

The family's journey to honor Leslie's final wishes (a Buddhist cremation) leads to a confrontation with her traditional Christian father, Jack (played by Frank Langella). Production and Reception Captain Fantastic (2016) - IMDb

It questions the ethics of imposing radical beliefs on children, especially when those beliefs hinder their ability to function in society.