Debra Granik is the exceptional film-maker who directed Winter’s Bone in 2010, launching the career of Jennifer Lawrence, and now she returns with this deeply intelligent, complex, finely tuned and observed movie, adapted by Granik and her screenwriting partner, Anne Rosellini, from the novel My Abandonment by Peter Rock. Their new title alludes the rules of respect and care for the environment promoted by ecological campaigners: to minimise human impact on nature.

Weirdly, this film initially reminded me of a fatuous and naive (and bafflingly overpraised) film called Captain Fantastic, which features Viggo Mortensen as a charismatic, disciplinarian dad who has taken his children to live with him in the wilderness. Leave No Trace is everything that that movie should have been: careful, realistic, with a sense of what is possible and what is at stake for those people who really do attempt to turn their backs on conventional living and also reject the stigma of homelessness — but what is also at stake for their children who have had no choice in the matter.

Ben Foster and Thomasin McKenzie play Will and Tom, a grizzled army veteran and his 13-year-old daughter. The question of Tom’s mother is not addressed. Will and Tom are living a kind of radical Thoreau-guerrilla existence in a huge public park in Portland, Oregon. They have built a secret camp with tarps and rudimentary cooking implements, making their own fires. They share a tent. They read books. They have military-style drills for staying undercover. Periodically, they amble out of the park and into the city, where Will can pick up his prescription for opioid painkillers at the vets hospital, which he can discreetly sell for cash on the black market to buy food — and then they return to the jungle. It seems like a perfect, even Edenic setup. But then Tom carelessly allows herself to be spotted by a hiker and things take a wrong turn.

The personae of Will and Tom are strikingly restrained, both in their conception and performance: there is an attractive humility and restraint at work, a quietism. No scenery-chewing, no fireworks, no obvious scary-Colonel-Kurtz stuff from Will or obvious teen rebellion histrionics from Tom. Neither appears concerned with what the future holds for them, nor when Tom should really be getting a tent of her own – let alone meet other people her own age.

When they are picked up by the authorities, they are subject to very similar psychiatric assessments, in which they have to respond true or false to questions about whether they have dark thoughts, etc. In some ways, these tests are callous, soulless – precisely the kind of bureaucratic intrusion that Will has passionately rejected on his own behalf and that of his daughter. And yet it is clear that this is the first time either of them have considered these questions – the first time they have really thought about themselves. Perhaps not having to think about yourself, not having to shoulder the burden of relentless neurotic self-examination, is part of what their way of life is about.

Interestingly, getting picked up and then escaping is also part of their way of life. They have clearly planned for what happens. They have to accept – or pretend to accept – the social services’ remedial plans for them before they can slip away once more. There is a great sequence in which they attend a church service, blandly complaisant, not making a fuss, not standing out, biding their time. Or rather it is Will who is biding his time; Tom isn’t so sure. Each time away from the wild brings Tom into contact with a society that she rather likes. Poignantly, she loves the rabbit that a neighbouring farmer’s kid has, and there is a lovely scene where she attends a rabbit-training school. Later, she has the same connection with honeybees. A split is coming. But Granik manages this crisis with cool, unhammy clarity. The intimacy and love between Will and Tom is presented with real delicacy. It’s a movie that will live with me for a long time.

