Nuclear-apocalypse survivor Ann Burden (Margot Robbie) lives a rustic existence on her family's isolated farm, protected from the dangers of a radioactive world by the surrounding hills and a plentiful water supply. One fateful day, Ann's life takes a turn when she meets John Loomis (Chiwetel Ejiofor), a refugee who claims to have escaped a nearby government bunker. Despite his initial sickness from contaminated water, Ann tends to Loomis, providing him a warm welcome in her humble abode.
As Loomis recuperates, he becomes a pivotal part of Ann's daily routine; together, they work on the farm, ensuring it thrives for the harsh winter ahead. Loomis shares his experiences, revealing his previous life as a research engineer trapped in a subterranean facility while the apocalypse unfolded. He helps Ann with practical tasks, including repairing the farm’s tractor and generating ideas for sustainable power. However, Ann is hesitant about Loomis' suggestions regarding the water-wheel powered by the nearby waterfall, citing her father's legacy as a preacher and her strong Christian beliefs.
Their bond deepens as they cultivate crops and plan for a shared future. Yet, underlying tensions begin to surface, notably due to Loomis' struggles with alcohol and their clashing perspectives on faith. Just as they approach a pivotal moment in their relationship, Loomis steps back, expressing the need for more time to navigate their emotions.
The dynamic shifts dramatically with the unexpected arrival of Caleb (Chris Pine), a fellow survivor who complicates matters further. Although Ann is welcoming, Loomis harbors resentment against Caleb, driven by his own prejudices and jealousy. The trio's relationships become entangled as Caleb and Loomis share their grim experiences, revealing the harsh reality of life after the bombs fell. Loomis opens up to Ann about a devastating memory that intertwines with her past, creating an emotional strain that clouds their growing attachment.
As Caleb advocates for the water-wheel project, the tension mounts, leading Loomis to give a reluctant nod for Ann to pursue a romance with Caleb, masking his heartache behind a facade of acceptance. Following a celebratory gathering, a drunken confession from Loomis breaches the surface of buried feelings, only to be overshadowed by Ann's impulsive choice to find solace with Caleb.
Their sexual encounter escalates the conflict between Loomis and Caleb, culminating in a dangerous moment during the construction of the water-wheel. Loomis returns to the farmhouse alone, fabricating a tale about Caleb's departure, which leaves Ann devastated. The restoration of electricity brings both relief and a sense of unease, as Ann discovers Loomis has taken the liberty of moving her cherished church organ into the barn. In a poignant moment, she plays a hymn, casting a wary glance at Loomis, who sits in silence, hands clasped, their fates intertwined as the scene draws to a close.
