PARK CITY - Until this week, I didn't even know there was a book called "Z For Zachariah," much less that it was by the same author as the wonderful "Mrs. Frisby and The Rats Of NIMH."

When we posted a clip from the new film adaptation of Robert C. O'Brien's book, it immediately became clear from your reactions here and in e-mail that the book has passionate fans, and that many of them were upset by what seemed to be a whole new character invented for the film. I couldn't respond because I don't know the book at all, and to be honest, what matters to me is whether the film works on its own. You don't need to know a book to know whether or not a film plays, and in the case of "Z For Zachariah," the film most definitely plays.

Craig Zobel, whose previous films "Great World of Sound" and "Compliance" are both smart and subtle storytelling, is working here from a screenplay by Nissar Modi, and he's once again made a sensitive movie with outstanding character work and smart attention to detail. If there was any question at this point, Margot Robbie's work here establishes her as one of the very best actresses in her age range today. I've had at least four conversations tonight with people who loved her in this and had absolutely no idea she was the same actress who played The Duchess in "Wolf Of Wall Street." In this film, she plays Annie, a young woman who finds herself completely alone after the world has seemingly ended. Because of the specific conditions of the valley where she lives, she's able to live comfortably. She lives on a farm where she can raise crops, and she's adapted to the solitude as best she can.

All that changes the day John Loomis comes wandering into her valley. Chiwetel Ejiofor is always good, but from the moment he slips off the radioactive safe suit we see him in at first, tears of joy at the mere existence of clean air spilling down his cheeks, it's obvious that he is giving this one everything he's got. Loomis is the first sign of the outside world that Annie's seen in quite a while, and at first, she just watches him, not wanting to approach. She has to step in, though, when he plunges into a pool at the base of a waterfall, because that water is coming in from outside the valley and it's still incredibly radioactive. Loomis ends up becoming violently ill, and it's up to Annie to keep him alive. Little by little, she's able to heal him, until finally he is up and around and fully aware of just what a paradise he's discovered.

For the first half of the film, the focus is entirely on the relationship between Loomis and Annie, and both of the actors are exceptional. Annie's been sheltered in more ways than just where she lives, and she is devoutly faithful, while Loomis has already faced some awful truths about survival and seems to have no use for God at all. There is a slowly-developing bond between them that doesn't play as a cliched romance. Instead, it's more about the gradual recognitions of things that they need or want. Loomis knows that it makes sense for him to be with Annie, just as she does, but he's also keenly aware of their age difference and of how little experience she's had with the world. He's careful with her, at least until he finds some beer in one of the coolers at the valley's long-abandoned general store. While drunk, he shows a side of himself to Annie that gives her pause.

Things become far more complicated when another survivor wanders into the valley. Caleb (Chris Pine) is in rough shape when he reveals himself to them, and at first, he talks about quickly moving on in search of more people. Once he's had a look around, though, and he sees what sort of life they're building for themselves, Caleb begins to make himself part of that. One of the things that "Z For Zachariah" gets right is the way jealousy has a half-life of its own, slowly burning you from the inside out. I used to say that I didn't understand the point of jealousy, and I judged people for it. Last year, though, I finally learned what it is first-hand, and the horrifying slow rot of it is something I hope I go the rest of my life without feeling again. There is nothing good about it. It is a poison, and while Annie's valley manages to avoid the effects of the war that destroyed the rest of the world, it is jealousy that finally manages to pierce whatever it was -- faith, weather, or pure dumb luck -- that kept Annie so safe for so long.

Judging by some of the conversations overheard outside the theater, "Z For Zachariah" may be too subtle for people who want big giant fireworks from the end of the world. The last ten minutes of the film are like a velvet sledgehammer, so subtle you might miss it, and so painful that it's hard to shake. Zobel works magic as a minimalist, and he knows that there is no special effect that can equal what happens when you're in close on a great actor who has something great to play. The film winds down quietly, but there are huge, permanent things happening just below the surface. Heather McIntosh's score is the perfect emotional counterpoint to Tim Orr's photography, which manages to capture both the idyllic nature of the valley and the way something darker slowly seeps in.

One of the things that I liked most about the film is that Annie is the lead, and by far the most complex character on the page. While her feelings towards the two men are important to the story, she's got more than that going on. Robbie seems to have a wicked knack for accents, but her transformation here is more than just a different hair color. As ferociously confident as The Duchess was, Annie is tentative, unsure how to read anyone. There are so many remarkable little grace notes to her work here, and both Ejiofor and Pine seem to be perfectly tuned to the work she's doing, playing off of her beautifully. Pine is a fascinating snake in the garden, gently manipulating both Loomis and Annie, and he uses his innate charm as a weapon here.

Ultimately, there's a delicacy to Zobel's approach that keeps this from feeling like a slam-dunk. It's easy to walk out of a film at Sundance like, say, "Whiplash," where you're basically floating because of the way things wrap up, and declare the entire film amazing. It's harder when you end on a quiet note, and Zobel refuses to serve things up easily. Still, I think this one's going to linger, and all three of the actors are very good in the film. "Z For Zachariah" may not be a faithful adaptation of a well-liked book, but as a film, it is a lovely, powerful piece of work.

"Z For Zachariah" will be released later this year by Lionsgate.

A respected critic and commentator for fifteen years, Drew McWeeny helped create the online film community as "Moriarty" at Ain't It Cool News, and now proudly leads two budding Film Nerds in their ongoing movie education.