Z for Zachariah. It's a loose (extremely loose , I think) adaptation of the 1974 novel by Robert C. O'Brien.
In a nutshell the story goes like this: nuclear war or accidental nuclear tragedy have left the world - well, maybe the world, but at least the USA, mainly unpopulated due to widespread radiation, and perhaps other ecological calamities.
Some little time has passed since the apocalyptic events. We meet a single survivor in a closely sheltered valley somewhere in eastern USA. We learn that this small valley area has missed the devastation of the rest of the world - or USA, due to a quirk of nature and its unusually sheltered location. There's a clean water supply. Suspension of disbelief is essential here because - what about when it rained ? The hard/ contaminated rain would have fallen there as well as everywhere else.
That aside, we meet a young woman, probably in her twenties, sturdily managing to survive working the small farm of her lost parents. She has enough food from crops, chickens and a cow, a little hunting and fishing to nourish her, clean water from a source not affected by outside radiation, and a faithful dog for company. Winter, though had been hard - without electricity after her generator ran out of fuel, she almost froze to death.
We meet her (Ann) in warmer times, as she is out hunting rabbits, and stumbles into another survivor (John), a black guy mabe a decade or so her senior, bumbling around in a huge hazmat suit with a laden trolley. She watches as he goes to bathe in a pool whose source she knows for sure is contaminated. She manages to alert him, but is too late, then tries to help him survive. Turns out he's a scientist, a quiet type, not easy to understand, but kindly and helpful to Ann.
That's a sketchy idea of the first part of the film, later on a third survivor arrives (Caleb) another male, this a more local fellow, one who turns out to be quite different in nature from John - a little sneaky, generally untrustworthy but not without some native charm.
We know, without benefit of synopsis, that this menage a trois will pose a problem, even in these terrible "end of days" circumstances, human nature remains human nature!
Z for Zachariah can be watched, as we did, head on (as it were), but after I'd read a little on line the next day from past viewers, it turns out there's a whole other way of seeing it: as an analogy. I had suspected there was an underlying Adam & Eve thing going on, but it goes deeper than that. There are biblical references, analogies, hidden hints, and important matters left unfinished, unexplained. One reviewer describes the movie as being a kind of Rorschach test. I agree, after having read through several lengthy threads of commentary about the film. There's mild exploration, if one looks for it, of issues connected to race, gender, class, religion, but in the kind of circumstances we all hope never to encounter.
I enjoyed the movie, but it's a definite slow-burn, needs patience, and maybe even a second viewing to appreciate all possible meanings. Just three characters support it (and a dog) - not a movie to attract those who enjoy lots of fast action, glamour and wall to wall noise. The acting is good to excellent, with Margot Robbie, Chiwetel Ejiofor, and Chris Pine it would be wouldn't it? With a trio of lesser actors this film could have been as disastrous as the times it depicted.
As for the original novel of the same title (which is a nod to a children's book: "A for Adam"), Wikipedia's synopsis tells me that the film adaptation uses nothing but the barest bones of the original story. The novel's tale sounds even harsher, goes where the film declined to go. Ann was much younger in the novel, teenage. Book version John had a rather nasty, controlling nature, wasn't black; and the novel introduced no third party to the scene. The film then has to be regarded simply as a stand-alone piece using a situation and location based loosely on the novel.