Although I’m not a parent, I’ve often thought about what it must be like to have the responsibility of a life put into your hands, a tabula rasa waiting to be filled in. There’s so much in the world that you would want to protect a child from, and so many good things you would rather they have in their heads: philosophy and physics and literature instead of video games and TV shows and internet porn.

Ben (Viggo Mortensen), the protagonist of Captain Fantastic, is one of those parents. To raise their six kids free of the damaging effects of the world, they took them deep into forest in the Pacific Northwest. In a house they built themselves, they raised them with no electricity and schooled them themselves. They taught them to hunt and live off the land, to be strong and self-reliant. What they can’t provide for themselves they buy with money from selling handmade crafts at a nearby general store.

But there was something else behind Ben’s plan to raise his family like this: He was also hoping to deal with his wife’s bipolar disorder. When she dies, in a hospital out of their realm, Ben has to bring his brood into the world for the first time to attend the funeral his rich in-laws have arranged.

Captain Fantastic (if the title refers to the Elton John song, I can’t see the connection) was written and directed by Matt Ross, the actor you may know as internet entrepreneur Gavin Belson on HBO’s Silicon Valley. He also co-starred as Alby Grant, leader of a polygamist Mormon sect on the HBO series Big Love, and I wouldn’t be surprised if working on that show gave him the idea for this film. It asks the question, to what extent can you be in the world but not of it, and is it even a good idea to try?

It’s an intriguing subject for a drama, maybe even too much so: Despite a running time of nearly two hours (which zip quickly by), you come away wishing that there was more to it. As befits a character played by Mortensen, Ben is almost supremely capable, so much so that we wonder about his background. It’s a shame that the children’s mother is out of the story before it even begins, as she represents tension in the world Ben has created that he is unaware of, at least until his kids come up against their wholly average cousins on the way to their destination. The ending is a bit abrupt and unsatisfactory, not least because it leaves us wondering what will become of this family in the years to come.

Still, it’s only a recommendation to note that a movie stays with you after its running time has ended. Captain Fantastic opens Friday at the Amherst and Eastern Hills Theaters.