We’ve long loved tales of surviving the apocalypse, whatever form doomsday takes — viral, nuclear, asteroid, aliens or zombies. But few stories built on theme embrace the dehumanizing fatalism of living on when everyone around you is dead better than “It Comes at Night.”

Trey Edward Shults’s thriller does what all such genre narratives do — show us the nuts and bolts of living on after civilization has collapsed. Then it shows us the psychological trauma  brought on by paranoia and violence, both of which you have to possess in an “us or them” life-and-death scenario.

We experience that through the eyes of Travis (Kelvin Harrison Jr.), a teenager and only child of Paul (Joel Edgerton) and Sarah (Carmen Ejogo). They’ve escaped the contagion that wiped out the cities, taking refuge at Sarah’s father’s old farmhouse in the forest.

And when we meet them, Sarah is urging her father (David Pendleton) to let go.

“You don’t have to fight it.”

His breathing is labored. He’s covered in buboes (bubonic plague is never mentioned). And they’ve confined him to a DIY contamination containment room covered in plastic sheets.

Travis is about to lose his grandfather, right before his eyes. Adding to the trauma, he’s going to have to help dispose of the body after Dad carries out a mercy killing. Try to get a good night’s sleep after that.

So even though they have each other, the safety of isolation, a decent food supply, fresh water and solar powered battery chargers, there’s a price to pay for living on.

That becomes clearer when a man (Christopher Abbott) breaks into their plague-free fortress. Travis sees what his school teacher Dad is willing to do to keep them all safe. But with his son a constant reminder of the humanity they’re barely clinging to, Paul takes a chance.

He won’t leave the intruder he’s tied in the woods, waiting to die. He’ll trust the guy’s story — that he has a wife (Riley Keough) and child nearby, that all he wanted was food and water, that his family is disease free.

As is the way of such outpourings of compassion in the movies or “The Walking Dead,” we can guess how this will be rewarded.

Shults has concocted a nightmare within a nightmare, a test of nerves and a wary mystery. Every wrong move, every falsehood uncovered has Paul alarmed that he’s made the wrong choice. The violence, when it comes, is both sudden and shocking, and seemingly justifiable, once we’ve thought about it.

Edgerton (“Animal Kingdom,””Black Mass”) ably gets across Paul’s desperation, his eagerness to find hope and something closer to the old “normal” in the horror that is his family’s new existence. But he’s acquired the skills and the absolute zero-tolerance mindset of a man who knows their lives have no room for mistakes, no margin for error.

Harrison has to play a kid confused and scarred by this new normal, raised to be kind and generous, but also impulsive and reckless, a hormonal, curious teen who can’t help but notice the pretty wife that’s just been added to their compound, a night-stalking boy eavesdropping on frank adult conversations about their chances and their motives, curious about adult sex lives as well.

The story’s no frills minimalism leaves Ejogo (“Selma”) too little to play, and packages too much of what we see — disease, blood, betrayal and death — as the boy’s nightmares.

But “It Comes at Night” recalls the dystopian visions of earlier eras, when the big question facing those who have survived the opening salvo of nuclear Armageddon was not “How do we live on?” but “Why?”

MPAA Rating: R for violence, disturbing images, and language

Cast: Joel Edgerton, Kelvin Harrison, Jr., Carmen Ejogo, Christopher Abbott, Riley Keough

Credits:Written and directed by Trey Edward Shults. An A24 release.

Running time: 1:37