What if you were the last man on earth?

Okay, now what if you were the second-to-last man on earth?

Craig Zobel’s “Z for Zachariah” is a fascinating film that I guess should be called science fiction, although there’s nothing futuristic, or even scientific, really, about it. Maybe the better term is “speculative fiction,” because it invites the viewer to speculate how they would react in a similar, extreme circumstance. What would I do? Who would I be?

A vaguely detailed apocalypse (possibly nuclear war, as radiation is a big concern) has wiped out seemingly all life on earth. But in an idyllic valley somewhere in the South, a woman survives. Her name is Ann (Margot Robbie), a devout farmer whose homestead has somehow been spared from the radiation. Now she is alone.

As we watch her forage for supplies in a neighboring town, fix fences, plant crops, Ann seems like an eminently capable woman. But then Loomis (Chiwetel Ejiofor), stumbles into the valley. She saves him from radiation sickness, and they bond. He is a man of science and she is a woman of faith — there’s is disagreement over whether to tear down the wood from her father’s chapel to build an electricity-generating water wheel.  But they are both human beings above all. Now there are two.

And then another man, Caleb (Chris Pine) arrives in the valley. He’s handsome, polite, but his motives are murky. He also claims to be a person of faith, and sees the fault lines between Loomis and Ann over this. Now there are three.

And three is a lot more complicated that two. Writer-director Craig Zobel, who made a horror movie out of two people talking on the phone (“Compliance”) is able to suggest much through dialogue and glances as a subtle power struggle (and perhaps a love triangle) ensues. What we’re not sure of is, in this new Garden of Eden, who are Adam and Eve, and who is the Serpent?

The performances here are terrific and layered. Pine, usually the action hero, makes his steely frat-boy looks seem untrustworthy as he carefully drives a wedge between the other two. Ejiofor is wonderful as always, playing Loomis as a good man who has suffered much in this new world, leading him to make sometimes rash decisions, pushing Ann away just when he shouldn’t.

And the Australian actress Robbie, who I didn’t think much of as Leo DiCaprio’s wife in “The Wolf of Wall Street,” completely sinks into the role of Ann, perhaps the strongest person of the three, but also the loneliest, and the one with the most to lose.

It’s a simple, almost elegant premise, and Zobel never puts a foot wrong in letting it play out, right up to a conclusion that feels somewhat open-ended. Zachariah was the father of John the Baptist, but other than that I don’t quite see the significance of the title, other than the scene in which Loomis finds a Christian children’s book on Ann’s shelf called “A is for Adam.” Perhaps the title means she is on the last page of her old book, trying to decide who she will start it over again with.