Starring Viggo Mortensen. Rated PG

“I’m a Trotskyist, Dad, not a Trotskyite,” says one of the high-strung free spirits in the winning Captain Fantastic. “Only Stalinists call Trotskyists Trotskyites.” That exchange tells you just how far off the grid you’ll go to find Viggo Mortensen’s bearded Ben and his six home-schooled kids, ensconced in a giant yurt somewhere in rural Washington.

When not reading Dostoyevsky or advanced biology texts, the three boys and three girls—all with unique names—are kept busy on an obstacle course provided by Mother Nature, since Ben has decided they should be able to stalk and skin a deer as easily as they can play guitar or discuss quantum theory. Their own mother is in a hospital somewhere, for reasons gradually explained. But volatile family dynamics are only heightened by her absence. Eldest son Bodevan (Scotland’s George MacKay, unrecognizable from the Proclaimers musical Sunshine on Leith) is Dad’s biggest admirer, although he’s secretly been applying to Ivy League colleges. Younger brother Rellian (Australian Nicholas Hamilton) is the quiet rebel, chafing against Ben’s autocracy and, tellingly, his taste in music.

The mom’s situation eventually pushes this backwoods clan—ignorant of pop culture and social norms—onto their rickety, book-lined bus (called Steve) for a trip to New Mexico and a volatile culture clash with her wealthy parents (Frank Langella and Ann Dowd), who force gentle Ben to face some home truths about the limitations of his seemingly enlightened patriarchy.

All of this is beautifully observed by writer-director Matt Ross, making his second feature after establishing himself as an actor best known as sly techie Gavin Belson on Silicon Valley and Chloë Sevigny’s creepy brother on Big Love. The cast is note-perfect, even if Ross is less sure when differentiating the other kids, particularly the redheaded sisters played by teens Samantha Isler and Annalise Basso. The film’s two unpredictable hours are also burdened by a couple of dragged-out false endings, before hitting a finish that, like many things in life, is all the more satisfying for the uncertainty that led to it.