Captain Fantastic begins in a paradise based on Plato’s Republic. Ben and Leslie, attempting to escape from Leslie’s mental illness, have retreated into the wilderness of the Northwestern United States, where they and their children live isolated from the rest of the world—isolated from greed, ambition, and deceit, from all that is broken in the world. There, in the Eden they’ve created, they live out what they think it means to be truly free, giving their children an intense physical and intellectual education and teaching them to live off the land.
But Leslie’s illness isn’t cured by paradise, and while in a hospital being treated for the only brokenness she and Ben could not escape, Leslie commits suicide. Leslie’s father forbids Ben from attending the funeral, but Ben is quickly reminded by his daughter Zaja of the highest ideal in their Eden: human freedom.
Together, he and his children venture back into the world from which he and Leslie escaped, and in the clash that ensues between Ben and Leslie’s (extremely wealthy) family, Ben questions both his ability to parent and the education that he’s been providing his children in the wilderness. Ben even comes to question the wife he thought he knew when he learns that she helped their oldest son Bodevan apply to elite colleges like Harvard and Stanford.