“LEAVE NO TRACE” — 3½ stars — Thomasin McKenzie, Ben Foster, Jeffery Rifflard; PG (thematic material throughout); in general release

As “Leave No Trace” opens, a veteran named Will (Ben Foster) and his 13-year-old daughter Tom (Thomasin McKenzie) seem to be on a long-term camping trip. A lush green forest surrounds them and we watch them operate day by day in their humble camp, preparing meals and maintaining their modest accommodations. We don’t know how they got there or how long they've been there, but we know Will is suffering from nightmares and his daughter is struggling to remember her mother.

Debra Granik's film takes us through the pair's routine on civilization's fringes, which involves periodic expeditions into nearby Portland to visit the VA hospital and buy groceries. Will sells his PTSD medications to a larger camp that shares their state park home, and he and Tom take great pains to avoid discovery from maintenance crews and passersby.

A team of police and park rangers shatters this quiet existence, whisking Will and Tom back into civilization for evaluation and testing. The state can’t let them return to the park, so instead finds a generous man named Walters (Jeff Kober) who will offer them a modest home in exchange for working on his Christmas tree farm.

This turn of events triggers a split that will develop through the film. Will visibly wrestles with anyone who tries to make him conform to a traditional existence and Tom gets a peek at a promising future as a local boy (Isaiah Stone) introduces her to programs such as 4H and Future Farmers of America. Though these opportunities intrigue Tom, Will forces her to pack her things in the middle of the night and they embark on a journey Tom begins to understand will have no true destination.

“Leave No Trace,” based on Peter Rock's novel “My Abandonment,” is a somber, heartbreaking and tender film that takes a unique perspective on PTSD. Early on, it’s easy to suspect Granik’s intention is to question the mores of society and to suggest that Will and Tom have every right to live and be happy the way they choose. But as the story moves forward, it becomes increasingly clear that Will’s idealistic notions are rooted in his personal struggles, and that what may be unique parenting on one level is threatening to do more damage in the long run.

Thanks to Foster's excellent performance, Will becomes a character that remains sympathetic and genuine even as he evolves into the film’s antagonist. There is never any doubt of his love for his daughter, and Foster makes his struggle very real. McKenzie’s understated turn as Tom may be even better, a young teen being forced to grow up way too early.

Overall, “Leave No Trace’s” exploration of powerful themes like the power of community and the kindness of strangers turns what appears to be a sad story on the surface into something very worthwhile. Granik’s film isn’t a traditional “happy ending” kind of film but rather an understated comment on the painful boundaries of genuine love.

“Leave No Trace” is rated PG for thematic material throughout; running time: 109 minutes.