Captain Fantastic opens in a riot of green leaves and trees. A deer appears before us, pauses, and behind it we see an indefinable area of grey flickering oddly to reveal a human eye. Seconds later, six woad-coated tribesmen appear from nowhere, slash the deer’s throat, gut it and hang it to drain. A boy is blooded. He eats some deer heart. Today he is a man, his father tells him.

As we’ll learn, this is a family who have lived off the grid since the eldest of six children – the blooded one – was three, 15 years ago. Patriarch Ben Cash (Viggo Mortensen) has raised them to fend for themselves and each other, to grow and hunt their own food, to protect themselves with or without weapons, and to question conventional wisdom.

So far, so survivalist/Swiss Family Robinson/Republican nightmare; but no, Ben is actually a Chomskyan libertarian leftist, and he puts no less emphasis upon his children’s intellectual development, educating them with everything he says and does. In one calm early moment that almost made me want to stand up and cheer, the whole family sit in silence around the fire, intently reading their books – hard books such as Middlemarch and The Brothers Karamazov. And it works: the kids are bright and curious, and the eldest, unbeknownst to his father, has acceptance letters from four Ivy League colleges hidden away, a ticking time-bomb for the family.

The first hour of Captain Fantastic keeps us in the idyllic forests of the Pacific Northwest, almost persuading us that Ben has done great work for his children, but there is a growing sense that he has taught them everything except how to live in the real world. When bad news arrives about the children’s mother, whose puzzling absence has been glancingly noted, the family must board their converted school bus and return, for now, perhaps for longer, to suburbia, to family, and to “responsibility”, however it’s defined. In the process, all the family’s contradictions and strains rise to the surface.

Captain Fantastic hangs on its central character – tough, wise, charismatic, arrogant, possibly deluded – and Viggo Mortensen, usually a very contained performer, is magically effervescent in his chieftain-patriarch role. But his stellar work is underpinned by a marvellously organic ensemble. Writer-director Matt Ross (who plays nerd-mogul Gavin Belson on Silicon Valley) has pulled off something remarkable here, a funny, clever, moving family drama with not one whit of cuteness or preciousness and no selling its characters short. I will watch absolutely anything Ross chooses to make next: a fine director is born.