Horror finds its footholds in uncertainty. I’ve always contended that the most effective horror films – for me, at least – exploit our fear of death. That can be realised in a few different ways: fear of physical injury or impairment, fear of oblivion or the pure fear of unknowing. Of what’s looming in the dark corners of our bedroom, behind that door, what awaits after we abandon this mortal coil.

It Comes at Night isn’t exactly a horror film; set in the wake of some unexplained apocalyptic plague, it marries the conventions of horror cinema with the ubiquitous post-apocalyptic subgenre. But director Trey Edward Shults demonstrates a sophisticated understanding of horror nonetheless. He exploits the familiar formal elements of the genre – suspenseful pacing interrupted by quick cuts; sets barely-lit by singular light sources; an ominous, occasionally overbearing score – to keep his audience on edge throughout his film’s economical 90 minute runtime.

What ensures the film’s effectiveness at scaring – or, at least, unnerving – its audience is its destabilising insistence on avoiding explanation. What is the ‘It’ that Comes at Night? Don’t expect easy answers to that question. Shults exploits a full spectrum of uncertainty, particularly when the isolation of the central family (Joel Edgerton’s Paul, Carmen Ejogo’s Sarah and their son, Kelvin Harrison Jr’s Travis) is interrupted by would-be ransacker Will (Christopher Abbott). Can we trust that he was merely trying to feed his wife Kim (Riley Keough) and infant child, or does Will have allies waiting in the woods? Is Travis’s attraction to Kim an innocent crush, or something more sinister? And what is the family dog barking at so insistently in the depth of the woods?

It Comes at Night’s aversion to explanation makes it a legitimately nerve-shredding experience. Judged on that criteria alone – ‘is it scary or not?’ – it’s an unequivocal success. But there’s a reason that most horror films – even the most successful ones – tend to collapse their anxious atmosphere with exposition midway through, because horror is about more than uncertainty. By delaying clarity until late in the piece, the film is defined by its twist – or lack thereof.

I’m reluctant to go into more detail, because unpacking the particulars of It Comes at Night’s third act risks erasing the real pleasures – or terrors – or the film. But suffice to say that the film lacks the robust subtext that separates good horror from great horror. The screenplay ultimately has little to say that you wouldn’t find in a middling episode of The Walking Dead. I walked out of the theatre with a feeling of dissatisfaction proportional to the apprehension I felt during the film.

The film is, nonetheless, a formidable showcase of Shults’ skills as a director. It’s clear, however, that he has room to grow as a screenwriter, but I’ll be there the next time he steps behind the camera.