A lone figure, covered head-to-toe in a makeshift hazmat suit, walks silently through an abandoned country town that’s somehow remained green despite the nuclear holocaust that has engulfed the planet. As this survivor removes a hooded mask, Z for Zachariah gives us our first glimpse at the naturally beautiful face and shoulder-length auburn hair of Ann Burden (Margot Robbie). Then, we discover that she’s not completely alone. As is customary in the post-apocalyptic film genre, she’s joined by a canine companion. Finished with a day of scavenging, this girl and her dog return to her picturesque farmhouse, where they dine by candlelight while relaxing to a vinyl record playing on an old hand-cranked phonograph.

Almost too quickly, Ann stumbles across another survivor near her home, this one wearing a real containment suit, struggling against the outfit’s bulk while carting a small supply wagon behind him. This new world’s getting crowded. Even Charlton Heston’s Omega Man enjoyed at least a reel of quiet to reflect on his solitude, but only five minutes in, Ann finds herself confronting John Loomis (Chiwetel Ejiofor), demanding that the frightened and paranoid man lower the shaking pistol he has trained on her. Less concerned by the prospect of being shot than for the welfare of the man threatening to shoot her, she begs him to climb out of the stream he’s bathing in. Although his Geiger counter had been giving him clean readings, prompting him to shed his sweat-filled “safe-suit” prototype, she informs him that contaminated water is flowing into the creek from a source outside this miraculously habitable valley.

From her accent, it’s plain Ann’s a Southerner, and since this is a movie, it’s also a signifier that she’s highly religious. John, who’s revealed to be a man of science, must therefore be atheistic. Evidently genre conventions need to be satisfied as much as belief systems need to be tested.

As John recovers from the radiation-induced vomiting that has left him a bedridden guest on Ann’s farm, he hypothesizes that the valley it’s nestled in has somehow produced a clean weather system, described as a “meteorological enclave” in Robert C. O’Brien’s science-fiction novel for young adults, 1974’s Z for Zachariah, a Jane Addams Children’s Book Award winner told through the diary entries of a 16-year-old Ann. O’Brien’s tale devoted a lot of ink to Ann’s writings about farming for survival, but the book becomes a different kind of survival story, with Ann ultimately fleeing from the much older John, who is gradually revealed to be a murderous taskmaster prone to sexual assault.

Here, the novel serves only as a springboard. Nissar Modi’s screenplay retains the character names and milieu, but transforms Ann into a survivor much closer in age to John, while the 25-year-old Robbie, a rising starlet whose looks are matched by her talent, plays her as the more sexually driven of the two. The overtly villainous aspects of John’s character are jettisoned, allowing director Craig Zobel to take the plot in different directions.

Ann’s still a damn good farmhand in this iteration, but she’s limited by a severe lack of resources. She has a tractor, but no gas to run it; a generator, but no working engine to power it. Ever the scientist, John demonstrates how to pump petrol without electricity and formulates a plan to make good use of the irradiated water that sickened him, hoping to use it to fuel a hydroelectric generator—if he can convince Ann to let him dismantle the church her father built and repurpose the wood for the project.

Ann admits to John that she almost didn’t survive the past winter due to hunger and cold; without the companionship of her dog, she might have given up. Now she finds herself energized not only by John’s ideas, but by his very presence. Unlike in the novel, John is the one who resists Ann’s advances, observing that it will “change things,” adding, “We have time.”

But do they? The next morning, the biggest departure from O’Brien’s book arrives in the form of Caleb (Chris Pine), a blue-eyed Christian who, despite some scruffiness, looks far too buff and healthy to have wandered in from the nuclear wasteland. Race enters the picture too, and the focus on moral and religious quandaries shifts to a rather tepid love triangle that appears nowhere within O’Brien’s blueprint.

In the end, the situation the characters face feels smaller than the cruel psychological experiment at the center of Zobel’s previous film, 2012’s claustrophobic Compliance, even though here the fate of the human race may hang in the balance. If jealous rivalries are all we have to offer, maybe we’re better off not surviving.

Z for Zachariah **

Starring Margot Robbie, Chiwetel Ejiofor and Chris Pine. Written by Nissar Modi, based on the novel by Robert C. O’Brien. Directed by Craig Zobel. At West Newton, Hollywood Hits and On Demand.