“It Comes at Night”

*

(R, 1:37)

You can watch “It Comes at Night” straight through and never know exactly what “comes at night.” Perhaps “it” refers to the fatal illness that is ravaging the world and that seems to have destroyed civilization. Or maybe it refers to something else — to people who show up at night, or to the bad things that happen at night.

In any case, this is a post-apocalyptic thriller. It’s a world in which the people remaining don’t quite know what happened. Taking stock, knowing things, weighing evidence — these are all the products of calm reflection, of breathing room, of a safe space for people to think. In “It Comes at Night” there is no such space.

As a portrait of what it might be like after the whole thing collapses, after systems carefully designed over centuries give way to bad luck or colossal stupidity, “It Comes at Night” is pretty convincing. Food and water are at a premium. People retreat into their families and trust no one on the outside, etc. There’s just one big problem here: “It Comes at Night” is about as enjoyable for the audience as it is for the people in the movie. On both sides of the screen, misery reigns.

Joel Edgerton and Carmen Ejogo are husband and wife, and Kelvin Harrison Jr. is their 17-year-old son, and they are holed up in a big house in the woods. They have access to water. They have guns. They have locks on entrances, and every window is boarded up. And then one day — at night, of course — a man (Christopher Abbott) breaks in, and the family must take defensive action.

That’s the initial event, and things build and expand from there, but they don’t build or expand very much. To the extent that “It Comes at Night” is a mystery, it stays a mystery. It depicts the characters’ predicament reasonably well — although the loud and noisy soundtrack is more annoying than pulse-pounding. But it doesn’t venture much past the premise.

Written and directed by Trey Edward Shults, it represents the second time in the past few weeks that a film with a lone writer-director has run into the same difficulty. Like Robin Swicord’s “Wakefield,” “It Comes at Night” has one good idea, and some smart development of that idea, but it pulls back just when you want and expect it to go ahead. There was simply more here to be gleaned, not just for the audience, but for the filmmaker. How did the world get like this? What does it mean? Is the situation irreversible?

It is not unreasonable for an audience to be interested in these questions when our interest is generated by the film itself. In the absence of such answers, or the intimation of such answers, or even of characters in pursuit of answers, “It Comes at Night” begins to seem thin, a torment without purpose. After all, the characters may be stuck in the world of the movie, but we’re not. We can still leave, for the time being.