Don’t let the publicity photos of the ensemble cast clad in ’70s-era tuxes and flower-child dresses, or even the cloying Mumford-mimicking soundtrack on the trailer, fool you: Captain Fantastic ain’t some twee, cutesy Wes Anderson romp or a Little Miss Sunshine knockoff. This dramedy marking the sophomore feature of longtime actor Matt Ross (who plays Gavin Belson on Silicon Valley) has the breezy Americana momentum of a hobo riding the rails and the dramatic chops of none other than Viggo Mortensen. If it weren’t for the godawful music and a few tonal missteps, this would be a near-perfect meditation on love, loss and the evolution of the American child.

Ben Cash (Mortensen) rides into town in his family’s converted school bus and finds out from his sister Harper (Kathryn Hahn) that his wife “finally” committed suicide while hospitalized near her parents in New Mexico. Ben delivers the news to his six kids as they sit in their survivalist bunks in the family yurt, but the grieving is brief — the family still has its “training.”

The Cash kids do physical drills and read advanced novels and philosophy texts every morning, preparing to be super-children, raised away from the consumerist American culture. The two eldest daughters, Vespyr and Kielyr (Annalise Basso and Samantha Isler), practice knife-fighting, then go home to dress the deer their oldest brother, Bodevan (George MacKay), killed with his bare hands. As expected, these people are prepared for a nuclear meltdown but not for the regular human interaction along the way to New Mexico for their mother’s funeral.

Great scenes emerge when the family ventures into the world, such as Bodevan’s tense first kiss in an RV park, or an awkward dinner with Harper’s family, where Ben and his kids don’t understand why Harper wouldn’t just tell her two boys that their aunt committed suicide by slitting her wrists. But Ross keeps a measured tone with a little humor to cut through the sadness, mostly accomplished by Mortensen in his sensitive performance as a dad who’s just trying his best and his stellar supporting cast.

MacKay’s facial expressions convey a multitude of emotions as he watches his cousins play video games for the first time, and even bit parts are elevated — Ann Dowd appears onscreen for only a few minutes as Abigail, but her interaction with her long-lost grandkids is pitch-perfect as she stumbles, shaking her head, distraught with a nameless grief.

The look of the movie doesn’t often fall short, with muted colors mirroring the quiet dramatics. But a few scenes deviate, such as one in which the family wears bright-hued garments and bursts into a church lined up behind their patriarch to defiantly attend their mother’s funeral; that’s the kind of “triumphant” (and derivative) scene that strays too far from Ross’ restrained palette.

Given its original premise and solid execution, Captain Fantastic’s music doesn’t jell at all. The portrayal of a good-hearted family in crisis that eschews all modern luxuries is mismatched with bland indie and commercial pop hits. It’s as if someone hit shuffle on an ordinary “cool dad’s” iPod and let it go. But Ben Cash isn’t a cool dad; he’s a sensitive, borderline-psychopath survivalist, and this endearing, original film is deserving of some equally antiestablishment anthems.

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