“Z for Zachariah” may not be the most eventful post-apocalyptic drama, but its grip is strong and sure. Set in a verdant valley (with New Zealand standing in for the American South) that has somehow survived the fallout from an unspecified nuclear catastrophe, this minimalist but deeply affecting morality play slowly tugs you in.
Its fulcrum is Ann (a mesmerizing turn by the young Australian actress Margot Robbie), a preacher’s daughter left alone when her family headed out to seek other survivors. Pale and purposeful, she grows vegetables and traps small animals, radiating the calm confidence of those accustomed to leaning on faith — unlike Loomis (Chiwetel Ejiofor), the research scientist who stumbles into her valley one day, unwisely strips off the protective suit he designed and jumps into an irradiated waterfall.
Nursing him back to health, Ann is drawn to his intelligence and enterprising spirit, each finding a renewed purpose in the other’s companionship. Sex and romance seem inevitable, and Ann is more than game, but Loomis, who exudes an innate decency, prefers to wait. Older, African-American and atheist, he may still feel constraints that should no longer apply.
This pairing alone is thematically fertile, but the director, Craig Zobel, is after more than just a joust between religion and rationality. And though the two belief systems butt heads when Loomis wants to tear down the church built by Ann’s father and repurpose the wood as a water wheel for hydroelectric power, faith-based conflict is not the film’s primary engine. That would be jealousy.
Adapting a 1974 novel by Robert C. O’Brien (posthumously completed and published by Mr. O’Brien’s wife and their daughter), the screenwriter, Nissar Modi, conjures an admittedly trite Eden metaphor — with the camera zooming pointedly on a children’s book titled “A Is for Adam” — rived by uneasily jagged undertones. If Ann and Loomis are our original sinners, then the snake is Caleb (Chris Pine), a rangy former miner who slithers, destitute and devastatingly handsome, onto their doorstep and into their heads.
Shot with shimmering naturalism by the cinematographer Tim Orr, “Z for Zachariah” responds to future doom with primal instincts. Race still rankles (at one point, Loomis comments that white people belong together), and the bruising realities of age are amplified. But repopulating a blasted earth requires commitment, not passion, and Mr. Zobel — as he proved with his profoundly unnerving 2012 thriller, “Compliance” — has no problem depicting sex as a destabilizing force used to exert superiority. Maintaining a strict formal allegiance to reserve and restraint, he shapes a dreamily elegant emotional ballet from glances and gestures and subtle shifts in power.
The film’s casting, like its aesthetic, is golden. Ms. Robbie is almost unrecognizable as the woman who played the brassy wife of Leonardo DiCaprio’s character in “The Wolf of Wall Street,” and Mr. Pine sways from cunning to innocuous with sly skill. Mr. Ejiofor, of course, never disappoints, and here he gives Loomis the beguiling vulnerability of a man carrying a dreadful secret — one that could alienate Ann for good.
From its magical setting (what gives this valley its environmental protection?) to its vacillating mood and chillingly vague conclusion, “Z for Zachariah” keeps you off balance. In wondering what will matter at the end of the world, the story neither excavates nor resolves all of its themes. It’s just as well that the pleasures of the journey are more than enough.
“Z for Zachariah” is rated PG-13 (Parents strongly cautioned). Add one curious virgin to one sexy drifter, and stand back.
Schedule information on Friday with a review of the movie “Z for Zachariah” omitted two of the five producers. They are Tobey Maguire and Matthew Plouffe.