Over and over again in It Comes at Night, Trey Edward Shults’s bravura piece of psychological horror, the camera pans down a long, dark hallway, inching closer and closer to a locked red door. That door is the only way in and out of the boarded-up house in which the film is set; behind it lurks the unknown. It Comes at Night is set in a world gone to rot, where vague nightmares might be waiting outside; it’s a film that plays on fear, rather than monsters, and explores how that kind of grand, unknowable fright makes people behave, well, monstrously.

Shults’s debut film was 2015’s Krisha, a marvelous, micro-budgeted family drama that played like an intense horror movie; following a long-estranged mother returning to her children for a Thanksgiving dinner, it racked up tension better than most films can manage with impressive set pieces and ample resources. Compared to Krisha, It Comes at Night is a blown-out extravaganza, but true to Shults’s previous work it’s an intimate, domestic tale that generates scares by focusing in on the littlest details, like that looming red door. It’s an incredibly well-made film, but it’s not quite the thrill ride it might appear from its promotional material, leaning as it does on slow-building mood and a general air of mystery rather than sudden jumps or moments of gory violence.

It Comes at Night takes place in an unspecified future where the world has been ravaged by an unnamed disease. All we know is that it’s extremely contagious, turns one’s eyes jet-black, and renders you basically catatonic; at the start of the film, we see Paul (Joel Edgerton), his wife Sarah (Carmen Ejogo), and his son Travis (Kelvin Harrison) take Sarah’s father, afflicted with the illness, out of the house to die. Paul’s family has hunkered down in a large house secluded in the woods, sealing off every entrance but that red door. Shults’s film is the story of what happens when something starts banging on the other side.

Paul quickly finds Will (Christopher Abbott), a young father scavenging for his family, outside the house and takes him in, along with his wife Kim (Riley Keough) and their young child. We never learn much about what is going on in the outside world, just that Paul and Will both know they need to defend themselves from it. Much as he did with Krisha, Shults zeroes in on the powerful, specific bonds of family and how they rope you into behavior you might otherwise think irrational; as the film goes on, both Paul and Will, as well as Sarah and Kim, make small, calculated decisions to protect their spouse and child should things with their new roommates turn sour in the future.

Needless to say, the tension mounts up very methodically. Not a lot actually happens in It Comes at Night—this is no episode of The Walking Dead, where zombies might start swarming at the windows. But Shults’s narrative is reminiscent of that TV show, and a thousand other dystopian stories, in that it’s really a tale about the terrible things people do when backed into a corner, and about the limits of human empathy. Every decision Paul or Will makes, no matter how horrifying, has a disturbing air of rationality to it. There’s little in the way of affection or common comforts, even when the two families are allied together, but the director manages to find something moving even in the grim acts of brutality that eventually erupt.

Though Paul, played with typical taciturn moodiness by Edgerton (this performance is a close cousin to his work in last year’s Loving and Midnight Special), is nominally the film’s lead character, the action is really told through his son Travis’s eyes. Harrison plays him as similarly quiet, but more open-hearted than his father, perhaps because he’s been a little more protected from the outside world. Still, he’s haunted by terrible dreams of death and disease. These dream sequences are both the creepiest, and the most visually impressive parts of the movie—while Shults tends to avoid the obvious scares one might expect from a work of horror, here he really indulges in strange, evocative imagery.

One imagines in a normal world Travis’s dreams could be given over to more winsome fantasies, and at times they briefly indulge in those typical reveries before being infected with gross, blood-dripping dread. That’s It Comes at Night—a fairly straightforward post-apocalyptic story, tightly focused on human torment, but suffused with surprising, undeniably atmospheric sights and sounds. Don’t go in expecting a high-octane thrill-ride, but do expect to emerge with details of the family’s house, and Travis’s dark visions, lodged in your brain; this might be a stripped-down view of a dystopia, but it’s a distinctive one.