As edgily inflammatory as his debut feature, 2012's hot-button "Compliance," was, director Craig Zobel strikes a more subdued key with "Z for Zachariah." A post-apocalyptic drama with precious few of the conventions often found within the subgenre, the film focuses squarely on its characters as they try to make do with their desperate times and hopelessly desolate setting. The screenplay by Nissar Modi (loosely based on the posthumously published 1974 novel by Robert C. O'Brien) is a minimalist but thematically dense affair for which wider audiences may not have the patience or open mind. Indeed, those expecting zombies or aliens or hunger games to enter the equation will be sorely disappointed. That is not the picture Zobel was interested in making, and his decision to approach the story from a more realistic angle sets it apart. As an exercise in teeming subtext and low-key yet ultimately affecting performances, it casts a delicate spell.

Months after a radiation pandemic wiped out most of the world's population, Ann Burden (Margot Robbie) lives alone with her dog at her family's West Virginia farm. Occasionally trekking to the nearest, now-abandoned town for supplies, she otherwise bides her time in the mountains, growing her own vegetables and wondering if she will ever see anyone—let alone her father and brother—again. When once more faced with humanity in the form of ex-government researcher John Loomis (Chiwetel Ejiofor), she is relieved to find someone else alive and welcomes him into her home. Their go at living together and the burgeoning attraction they share is rattled with the sudden appearance of another survivor, Caleb (Chris Pine). Using the wood from a nearby church, the trio begin constructing a water wheel in hopes of restoring an electric generator. When Ann finds herself connecting to Caleb on a deeper level, she is left torn by her unspoken allegiance to John.

"Z for Zachariah," which gets its name from a Biblical picture book Ann has called "A for Adam," is an exceedingly hushed chamber piece. Quiet tension looms in the expectation that something is going to happen—and it does, eventually—but director Craig Zobel stands true to the ruminative, unhurried tones of his characters' new way of life, virtually bereft of modern-day bustle and convenience. Discussions about religion, faith, honesty, loyalty and mortality evocatively coalesce within the fabric of a story where Ann, John and Caleb's values shift and revise themselves when faced with a world where, as far as they are concerned, they are the only people left. Chris Pine (2014's "Into the Woods") is underused as Caleb, arriving on the scene past the midway point and not given the chance to grow beyond that of a plot device—it is no surprise that this role was devised for the film and not featured in the book—but Margot Robbie (2015's "Focus") and Chiwetel Ejiofor (2013's "12 Years a Slave") deliver well-formed, exquisitely modulated turns. As Ann and John, they find nuances not even present in the script, their onscreen bond muddying and rising in complexity as long-hidden truths come to light. At times, it feels as if "Z for Zachariah" is holding back; one wishes the film wasn't quite so restrained and polite. The tricky ambiguity and tough suggestion of where the narrative leads, however, feels right.