The question of social conformity is at the center of Matt Ross’ “Captain Fantastic,” a manipulative feel-good dramedy about an unconventional family that coasts along likably on quirky writing and strong acting, even by the younger members of the cast. It’s only the second feature from Ross, who’s better known as an actor, and shows a considerable advance on his debut, 2012’s little-seen “28 Hotel Rooms.”

The film benefits enormously from the presence of Viggo Mortensen as Ben, a counter-cultural leftover who, together with his wife Leslie (Trin Miller), decided to live off the grid with their children Bodevan (George McKay), Kielyr (Samantha Isler), Vespyr (Annalise Basso), Rellian (Nicholas Hamilton), Zaja (Shree Crooks) and Nai (Charlie Shotwell). Unfortunately, Ben’s wife has fallen ill—she’s long suffered, we learn, from bipolar disorder—and has been sent away for treatment paid for by her disapproving father Jack (Frank Langella)—leaving Ben alone with the kids in their compound in a forest in the Pacific Northwest, where he teaches them self-sufficiency, survival instincts, physical skills and general knowledge through intensive home-schooling. It’s a peculiar but obviously loving, supportive domestic arrangement In which the youngsters might not have learned the usual social skills but embrace their own familial rituals, like celebrating Noam Chomsky’s birthday rather than Christmas.

Tragedy strikes when word reaches Ben of Leslie’s death by her own hand, accompanied by Jack’s insistence that his son-in-law stay away from the funeral. Ben will have not of that, especially since he’s aware of Leslie’s final wish to be cremated without any traditional service, and so he boards the clan on their hippie-like RV for a trip to New Mexico (and into the wider world more generally) to bid a final farewell to their beloved wife and mother.

The journey, of course, is an eventful one of new experiences and self-discovery. A visit with Ben’s brother (Steve Zahn) and his wife (Kathryn Hahn) and sons (Elijah Stevenson and Teddy Van Ee) shows the chasm that exists between Ben’s well-trained, but highly sheltered brood and ordinary American adolescents, and when the family shows up at Leslie’s funeral, it sets off a mini-war between Ben and Jack not only over the elaborate Christian service, which Ben dismisses as contemptuous of his wife’s explicit wishes, but over the future of the children. Even the irenic efforts of Ben’s mother-in-law Abigail (Ann Dowd) can’t resolve matters.

But the impact of the trip within the family unit is no less powerful. Eldest son Bodevan has his first quasi-romantic encounter, with Karin (Erin Moriarty), a flirty girl he meets at a trailer camp and, with notions of literary romance in his mind, at once believes he’s fallen in love with. He also reveals to his father the acceptance letters he’s received from prestigious colleges, to which he’d applied without telling Ben, though with help from Leslie. Meanwhile his younger brother Rellian turns against Ben, blaming him for his mother’s death and deciding to stay with Jack and Abigail. When Kielyr tries to intervene, she’s injured, and the episode leads Ben to question whether the kind of life he’s made for his children is preparing them properly for their inevitable encounter with outside society; he wonders whether it wouldn’t, in fact, be better to leave them in their grandparents’ care.

The nice thing about “Captain Fantastic” is that it aims for a degree of even-handedness: Ben is portrayed affectionately, but he’s not superdad, a sitcom figure with all the answers. Mortensen manages to embody the character’s contradictions, a quality of severity along with an innate gentleness that underlies his treatment of his offspring without condescension but with concern for their welfare. One might be taken aback when he gives his children, even the youngest, weapons as their presents on Chomsky Day, or refuses to help one of them when he slips while rock-climbing and injures his hand, or praises them for participating so efficiently in the theft of food from a supermarket. But one can also commiserate when he’s hurt by a verbal assault from one of the kids, and respect him when he apologizes to his brother for failing to abide by his house rules, or coaxes an intelligent response to Nabokov’s “Lolita” from one of his daughters who’s decided to jump ahead on her required reading list, or says a fond farewell to a son who’s chosen to leave the nest. Mortensen captures Ben’s varied moods and reactions with a quiet authority that never calls attention to itself, even when he’s wearing a flaming red suit to his wife’s funeral, or nothing at all in a scene at the trailer camp.

Ross’ skill is further exhibited in the fine performances he elicits from the younger members of the clan, with McKay, Isler and Hamilton the standouts among them. And while Langella can’t give much shading to rigid Jack, Zahn brings a nice note of fumbling brotherly love to his role. Among the technical credits, Stephanie Fontaine’s widescreen cinematography is particularly evocative, though Courtney Hoffman’s costumes add an appropriate touch of eccentricity to the wardrobe.

All told, “Captain Fantastic” is a heartfelt portrait of family values that may be unusual, but are admirable in their own way.