The classic scenario goes something like, If we were the last two people on earth…

In “Z for Zachariah,” based on the book by the same name, that’s sort of the case. And then another dude shows up.

But let’s back up. The film opens on the most gorgeous landscape. Mountains and fog, forests and rivers. In some deserted city, Margot Robbie collects books from a library and wears a version of a hazmat suit when she is outside. When she gets closer to home, she is able to take the suit off, seemingly okay outside in the open air as she hunts and, well, survives. It is just Robbie and her loyal pet dog.

Chiwetel Ejiofor arrives on the scene soon after in some sort of a vehicle and protective suit. It is clear that they are living in a post-disaster, toxic world. And it appears they are completely alone. Robbie lives in the house her father built, and we learn later that both her brother and father went out to find survivors after the disaster. For reasons unknown, the valley that the house rests in has remained untainted, unlike the surrounding lethal area.

Robbie meets Ejiofor when he is swimming in a waterfall, but he doesn’t realize that the water is toxic. She yells for him to get out, he doesn’t trust her. Both are deeply scared and cautious — because neither can believe they are actually seeing another human.

Director Craig Zobel creates an incredibly believable world. A world you might believe could exist if something terrible truly happened, and in their own subtle ways, we slowly learn how terrible it has been for both Robbie and Ejiofor as they have survived on their own, and inevitably lost many in the process.

Robbie, who is best known for her energetic performance in “The Wolf of Wall Street” as Jordan’s (Leonardo DiCaprio) wife Naomi, is much darker in “Z for Zachariah,” and it suits her well. Her hair is darker and her performance is one that will undoubtedly put the Australian actress on the U.S. map. In many ways, it’s her movie. We understand her loss the most, we feel her loneliness the deepest. We are most protective of her future.

The growing relationship between Ejiofor and Robbie is layered and complex, and then Chris Pine shows up. Add a second man to the equation and tensions are bound to multiply.

Zobel balances danger, suspense and the more delicate nuances of love expertly, leaving us unsure of both the threats outside the house as well as the danger within.

As the three characters work toward some type of rebuilding (Ejiofor is an engineer who is able to construct a water wheel and re-power the broken generator), they each have their own doubts and desires about how to exist in a world so different from the one they grew up in. When Ejiofor finds beer in the deserted convenience store down the road, things momentarily get dark. We see his darkness in bouts throughout the film but nothing is overt, until suddenly it is.

Robbie’s innocence and deeply embedded isolation make her perhaps the most complex character, and her desire to love and be loved attracts both Pine and Ejiofor into a post-apocalyptic love triangle.

“Z for Zachariah,” a film that everyone seems to be talking about at the 2015 Sundance International Film Festival, is one of the most compelling, if not commercial, of the independent films in Park City, Utah, this year. It’s bound to attract a “Hunger Games” older sibling audience, not to mention fans of “Children of Men” and “The Road.”