Nuclear-apocalypse survivor Ann Burden (Margot Robbie) lives an agrarian life on her family's valley farmstead, sheltered from radioactive contaminants by rocky hillsides, favorable weather patterns, and an abundant ground-fed water supply. One day, Ann encounters refugee John Loomis (Chiwetel Ejiofor). He claims to be an engineer, who, aided by medicines and a radiation suit, walked from a distant government bunker to Ann's valley. Loomis bathes in contaminated water, and immediately sickens, but is nursed back to health by Ann, who welcomes him into her farmhouse.Loomis regains his strength, and gradually, becomes part of Ann's humble rustic life. He helps Ann pump diesel from local petroleum pumps and gets the farm's long-disused tractor running to expand her gardens for the winter. Ann tells Loomis about her parents and younger brother who left the valley to find other survivors... but never returned. Loomis says he was a research engineer and stayed in a govt facility in a bunker, 1 mile below the surface. Thats how he survived the apocalypse. But then he had to venture out as he was running out of supplies. They need to stock up for the winter as last yr Ann almost starved and now there are 2 mouths to feed. She wants the freezers up and running, so they can catch and store fish from the local pond. Loomis speculates hydro-electricity might be generated from the nearby waterfall, using a water-wheel fashioned from the Burden church's planks and beams. Ann is uncomfortable with this proposal, citing her father's involvement as preacher and her deeply-held Christian beliefs. Loomis chooses to not pursue the project further.Ann and Loomis grow closer, cultivating crops and preparing for long-term habitation. Their domestic accord is marred by occasional tensions, notably involving matters of religion and Loomis' drinking. The two come to the verge of initiating a sexual relationship, but Loomis demurs, claiming a sexual relationship will change them, and he needs more time. Mysterious phenomena (including stolen food supplies and a half-glimpsed shadowy figure) culminate in the arrival of a third survivor, Caleb (Chris Pine). Although Ann welcomes Caleb into the farmhouse, Loomis resents Caleb, stating "whites belong with whites". Loomis questions Caleb's backstory (he worked in the mines and that's how he survived when the bombs went off) and motives; Caleb repeatedly emphasizes the religious connection he shares with Ann (in stark contrast to Loomis).The three survivors slowly settle into a marginally-stable partnership. Both men relate post-apocalyptic horrors they witnessed before reaching the valley; Loomis describes a radiation-sickened child begging him for death; later, he privately confides to Ann his belief the dying boy was her long-absent brother; Loomis confesses to the murder of the child. Caleb pressures Ann to go forward with the water-wheel project, and work commences on tearing down her father's church for materials. Perceiving their mutual attraction, Loomis awkwardly gives Ann consent to pursue a romantic relationship with Caleb, but belies his grief and anger in losing Ann through his remarks. Shortly thereafter, following a celebratory dinner, a heavily-intoxicated Loomis tells Ann he loves her before passing out in a bedroom in Ann's home. After failing to wake Loomis, seeming to want his affection, Ann joins Caleb in the adjoining bathroom, where the two engage in sexual activity.Further tensions arise between Caleb and Loomis following the sexual encounter. The two men finish the water-wheel, moving it and its wooden flume into place atop the waterfall. Encumbered by the bulky radiation suit, Caleb slips twice during his rope-assisted climb up the slick mossy cliff-side. During the second slip, the two men silently lock eyes, both holding the rope, while Caleb teeters on the cliff's edge.Loomis returns to the farmhouse alone. Ann apologizes for her earlier indiscretion; Loomis claims Caleb left in search of other settlements. Ann takes this news badly, chasing after Caleb but not finding him, and lapses into a sullen silence. The farmhouse's electric lights and refrigeration are restored. Ann realizes Loomis moved her beloved church organ and three pews into the barn. Ann, playing a hymn, exchanges a distrustful glance at Loomis who is sitting and clasping his hands, as the scene fades to black.