If I tell you "Z for Zachariah" is a post-apocalyptic love-triangle drama, you may presume it's some sort of half-cocked "Hunger Games" rip-off. But resist that urge, because this stealthy indie is nothing you'd expect and so uniquely thrilling that it's worth seeking out.

Based on the Robert C. O'Brien novel of the same name, "Z for Zachariah" centers on Ann (Margot Robbie) a farm girl who's become a woman on the fringe of a world ravaged by nuclear war. High up in her farmhouse, she's safe from deadly radiation, but alone, except for her dog Faro. Director Craig Zobel, who received praise for his challenging 2012 docudrama "Compliance," lovingly establishes not only Ann's routine, but also her loneliness. So what a deep pleasure it is for her (and us) when hope arrives in the form of a radiation-suited stranger named John (Chiwetel Ejiofor).

The two couldn't be more different: She's a naïve preacher's daughter who believes all this tragedy is part of God's plan; he's a jaded scientist who's seen too much horror in this toxic terrain to hold much hope for meaning. Yet after an initial wariness, the two begin to bond. That's when Caleb (Chris Pine) shows up, complicating this budding romance while proving an intoxicating enigma.

Zobel displays incredible confidence as a filmmaker, allowing the pace to be unhurried, focusing on the gradual transformations of his characters above all else. It makes for a thriller that is not so much about actions as reactions. A glance from Ann or a cringe from John can throw an entranced audience into anxiety. The film lets us get so close to these two, that their every emotion feels tied to our own. Our hearts are connected to theirs by a tender tether that tugs, creating brilliant pain and radiant joy.

Of course, all that would be for naught were it not for an exemplary cast. Each actor needs to be in top form to pull off "Z for Zachariah's" gamble. And Zobel couldn't have prayed for a better ensemble.

Robbie, whose profile raised when she was cast as Harley Quinn in "Suicide Squad," is unrecognizable as Ann. In both "The Wolf of Wall Street" and "Focus," Robbie was a leggy, blonde bombshell whose smirking smile promised sex and something more scandalously fun. But here, she's make-up free, with dull brown hair, and freckles that make her appear younger, softer and infinitely more vulnerable. Yet Robbie never plays Ann as a pitiable fool, or defenseless girl. Whether driving a tractor, hauling up well water or brandishing her rifle, Robbie seems at ease in this world, and defiantly joyful within her carefully lilting Southern accent. Ann is light and lovable, which makes us fear for her as these unknown men meander onto her turf.

Ejiofor, who won well-deserved praise for "12 Years a Slave," is superb as John. His pain and fear, made clear in his eyes, penetrate. His chemistry with Robbie is raw and wondrous, creating not just a sexual tension but also an emotional one. As the two are drawn together, John softens. Through this we not only get some warmly funny scenes of fumbled affection, but also perceive a chance for a world renewed. And as Ann's opposite, John establishes the two warring perspectives the film plays with: hopeful and cynical. Both of which come into play with Caleb.

He enters the scene as a shadow. And depending whether you're an Ann or a John, you might see him as a friend or a threat. At face value, Caleb is an honest, blue-collar guy who could use a home and a community. But if you read between the lines, you might make out an opportunist who'd say anything to survive. Pine plays this curious character with a delicate hand and a heat so intense it'd outrank ghost peppers on the Scoville scale. Last year, Pine offered a hat trick of compelling (and sometimes beautifully bonkers) performance between "Stretch," "Horrible Bosses 2" and "Into the Woods." Here, he continues this streak with his could-be creeper who's every word and expression is a pleasurable puzzle to be solved.

From start to finish, I adored "Z for Zachariah."

Its post-apocalyptic setting creates a playground for a finely tuned thriller about the paradoxical frailty and resilience of the human heart. Its cast has a heady chemistry that makes its fraught love triangle the most enthralling of any film in the past decade. It's so unmoored from the conventions of its genre that it gives audiences the all-too-rare joy of not knowing what could happen next. And the film's mind games demand you stay in its world long after the credits roll.

"Z for Zachariah" arrives  Friday in select theaters and on VOD.