As It Comes at Night begins, the disaster has already happened. This spare but savage horror movie, the second feature film from writer-director Trey Edward Shults (whose debut, the vertiginous addiction drama Krisha, made my 10-best list last year), takes place in the wake of some never-named catastrophe. Was the plague that seems to have killed off most of the Earth’s inhabitants triggered by war or environmental collapse? Did the world as we know it end with a bang or a whimper? With a reserve that’s rare in the era of elaborate world building, Shults never reveals the exact nature of this particular apocalypse.