If you have any doubts that Margot Robbie can act, or that Chiwetel Ejiofor is one of the very best actors of his generation, or that Chris Pine is a cut above your average Franchise Chris, Z For Zachariah will lay those doubts to rest. The three of them—the only three people in the entire film—are very good as maybe the last people on Earth, with Ejiofor going above and beyond and delivering a killer performance that is sure to be overlooked because this film is so small and weird no one will see it. It is worth seeing, though, and not just for the acting. Directed by Craig Zobel and adapted by Nissar Modi from Robert O’Brien’s young adult novel, Z For Zachariah is not your typical YA dystopia. It’s half religious allegory and half moral exercise, with a slow, measured pace that will test some viewers’ patience.

Robbie stars as Ann, a young woman who appears to be the sole survivor of some sort of holocaust—radiation and fallout are mentioned, so it’s likely the nuclear variety—and who spends her days working her family’s farm and trying to gather enough supplies to survive the winter. In the book Ann is a teenager—a critical character detail—but Robbie doesn’t look like anything less than a fully grown woman, even with neutral makeup and a brunette dye job. She seems clued into the dissonance, though, and works doubly hard to sell Ann as naïve and sheltered. It’s not the most natural performance, but it is for the most part convincing.

One day Ann encounters a strange man bathing under a waterfall near her home, and we start to learn some about what has happened to the world. The air in Ann’s valley is clean, but that stream flows from outside the valley and the water is radioactive. In one scene Zobel establishes how desperate things got before all the people died as both Ann and the newcomer, Loomis (Ejiofor), react with similar fear and panic at being confronted with another human. But Ann is trying to help because Loomis, exposed to the radioactive water, is immediately, desperately ill. And so she takes him home and nurses him back to health.

Zachariah is essentially a retelling of the Garden of Eden story, with Ann’s fallout-free valley standing in for Eden, and Loomis and Ann as Adam and Eve. As Loomis recovers, the two begin working together and getting to know one another, and Loomis quickly alludes to the possibility of the two of them having children and “saving” the world (how long do they think that would last with no one else to breed with…?), an idea that catches virginal Ann off guard. But the two are not well suited. Loomis is a man of science, Ann is devout. He’s older and far worldlier than Ann, whom he seems to regard with a mix of respect for her continued survival and impatience with her childish outlook, and she is put off by his mood swings and temper. And they don’t seem particularly attracted to one another, beyond the potential to produce offspring.

Enter the serpent, in the form of Caleb (Pine). He’s younger than Loomis, and he shares Ann’s faith and vaguely Appalachian background—and he’s white, too. This is where Zachariah loses a little momentum, as Zobel, whose previous film, Compliance, is a deeply unsettling look at abuse of power and sexual abuse, doesn’t bring the same unflinching perspective to this film. In one scene, upset at being cut out by Caleb, Loomis snaps, “Ya’ll go be white together,” which is a GREAT line that Ejiofor absolutely kills, but then it just doesn’t go anywhere. Zachariah presents a number of moral challenges, particularly how far one is willing to go to survive, with both Loomis and Caleb illustrating the harsh reality of survival in such an unforgiving landscape.

But Ann’s challenge, about how her rural, devout upbringing might be affecting how she views her two suitors, never materializes. It sidelines her, leaving her to be little more than an object for Loomis and Caleb to fight over. Those two follow their storyline about jealousy and competition to a logical, if somewhat ambiguous, conclusion, but Ann is just kind of there, in the end. She’s more mature, but she hasn’t actually chosen anything for herself. Or, you could take a darker reading on the third act and say that she did choose and the men vetoed her decision, but Zobel doesn’t commit enough to Ann’s position within the triangle to make much of a statement like he did with Compliance. Still, despite that oddly wishy-washy note, Zachariah is a complex and rewarding film, for those who don’t mind the glacially slow pace.

Z For Zachariah is in theaters and available on demand.