“Captain Fantastic” opens with a magnificent aerial shot of the treetops of the Pacific Northwest, a verdant, atmospheric prelude to the sensory plunge about to take place.

In the next scene, a young deer is warily making his way through some foliage; he’s being quietly observed by a young man who, within moments, will have captured and killed the animal. He is then joined by his five brothers and sisters who, like him, have slathered their faces in thick, tarlike mud.

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These young savages aren’t the feral creatures of a prehistoric era. Rather, they’re the sons and daughters of Ben (a superb Viggo Mortensen), the principled, adamantly independent nonconformist who is the film’s title character.

Ben and his wife, Leslie (Trin Miller), have been rusticating in the dripping woods with his six kids since the birth of their now-teenage son, Bodevan (George MacKay), whose slaughter of the deer is part of a primitive coming-of-age ritual. With Leslie in the hospital, George oversees a free-range brood of bright, curious, physically brave kids who are as comfortable with a boning knife as they are reading “Middlemarch” while wearing a gas mask.

That last touch is a nod to Ben’s mistrust of an outside world that, by his lights, is fatally commercialized, hypocritical and lazy. Writer-director Matt Ross vividly captures Ben’s overpowering influence on his children, who can’t help but come under his implacably demanding spell: When one of his daughters describes “Lolita” as “interesting,” he lights into her, accusing her of using a “non-word” and insisting that she provide a more nuanced, sophisticated literary analysis. Later, during the family’s annual celebration of Noam Chomsky Day, he gives his 6-year-old son a copy of “The Joy of Sex.”

The movie falters as it moves toward a mushy ending that the filmmaker can’t seem to bring himself to tighten up. Even with that hiccup, “Captain Fantastic” leaves viewers with the cheering, deeply affecting image of a dad whose superpowers lie in simply doing the best that he can.