I do not mind ambiguity, but the ambiguous nature of “Z for Zachariah” frustrates me instead of intriguing me. As a sort of post-apocalyptic chamber drama, the movie focuses on the psychological tension being developed among three characters who may be the last surviving human beings in their doomed world, and it surely holds our attention with its subtle tension accumulated below the surface, but how it eventually arrives at the finale is disappointing compared to what has been built up so compellingly during the rest of the film.

The movie opens with another lonesome day of Ann Burden (Margot Robbie), a young country girl living alone in her village which has been a safe haven for her since the collapse of human civilization. While it is not explained well in the film, it is implied that a global nuclear war broke out several years ago, and the world outside Ann’s village has been heavily contaminated by the resulting fallout while the village has been somehow protected thanks to its geographical and climactic conditions. Located in the middle of a wide field surrounded by mountains, the village has an abundant source of clean, uncontaminated water from the ground, and there are also other resources to help Ann maintaining her modest self-sustaining life.

Not long after her brief venture into a small empty town over the mountains, Ann happens to come across a guy named John Loomis (Chiwetel Ejiofor). John is an engineer who was luckily in an underground bunker when the war began, and he has been wandering alone while looking for any safe clean place like Ann’s village. Due to his unfortunate mistake during their first encounter, John finds himself getting exposed to a heavy dose of radiation, so he comes to stay at her house as going through a difficult recovery process.

As days go by, John gets better while showing his gratitude to Ann. He helps Ann manually pumping gasoline at a nearby gas station, and Ann is happy to use her tractor again for her farm works. John also gets a nice practical idea about how to generate electricity for them, but Ann is reluctant about his idea because that will demand a certain personal sacrifice on her side. She knows well survival comes first in her changed world, but that does not mean this good Christian girl can easily give up something important in her spiritual life.

And we sense the emotional attraction between Ann and John while they spend more time with each other under the same roof. After all, they are probably the last man and woman in their dying world, so it is natural that they find themselves drawn to each other despite their several differences, and the movie puts a little too blunt reminder on the screen when John looks around the library of Ann’s father which is full of religious books.

While Ann and John wonder about whether they should go further along with each other, a new situation begins with the unexpected appearance of another survivor coming from the outside. Caleb (Chris Pine) says he is going to a possible safe place somewhere in the south, and John regards this young handsome guy with reserved caution, but Ann lets Caleb sleep inside her house.

Not so surprisingly, the situation becomes more unstable as Caleb stays longer than expected. Ann and John are fine about getting extra help from Caleb during their farm works, but then Caleb begins to look less trustworthy as it seems he has some other thought in his head. Claiming to be a faithful Christian just like Ann, he approaches closer to Ann, and Ann does not seem to be so sure about which way her heart is inclined to. As the older man of the trio, John understands what may be going on between Caleb and Ann, and he is willing to accept whatever will inevitably happen – but then he finds himself grappling with how he feels about her.

The director Craig Zobel, who previously disturbed us with small but chilling drama “Compliance” (2012), did a good job of establishing the sense of desolation and isolation surrounding his three characters, and the cinematographer Tim Orr, who is David Gordon Green’s frequent collaborator, occasionally provides beautiful landscape shots while never overlooking the overall melancholic mood of the film. The three performers in the film are competent in their respective performances; while Chiwetel Ejiofor and Chris Pine make an effective contrast with each other, Margot Robbie, who has shown more of her talent since her breakout supporting performance in “The Wolf of Wall Street” (2013), holds the balance well between her co-stars.

Compared to the compelling set-up prepared by Zobel and his cast and crew, the finale of “Z for Zachariah” feels flaccid and underwhelming in its muted execution. I cannot go into details due to spoiler problems, but I can tell you that I was dissatisfied with how the outcome of one particular moment is clumsily left to our speculation without any substantial dramatic impact.

I am wondering about whether there is something lost in the adaptation process by Nissar Modi. I learned later that Robert C. O’Brien’s 1974 novel, which received the Edgar Award for Best Juvenile Mystery from the Mystery Writers of America after its posthumous publication in 1974, is different from the movie version in several notable aspects including characters, and now I feel more curious about the novel than before. Perhaps, it will give me a more palpable understanding of the title of the movie.

Sidenote: One of the other notable works by Robert C. O’Brien is children’s book “Mrs. Frisby and the Rats of NIMH” (1971), which was made into 1982 animation feature film “The Secret of NIMH”. I think you will probably have a better time with “The Secret of NIMH” than “Z for Zachariah”.