“Don’t trust anyone but family,” a father tells his son in Trey Edward Shults’ “It Comes at Night.” In the wake of a mysterious pandemic that has reduced a few survivors to a state of animal desperation, that’s sage advice. The father, Paul, played by Joel Edgerton, and his wife, Sarah (Carmen Ejogo), live in a rural cabin with their only son, Travis (Kelvin Harrison Jr.), who seems younger than his 17 years. When another family encroaches on their property, young Travis sees firsthand that his father means what he says.

Pandemic movies tend to start out the same — an outbreak, followed by societal collapse — but they distinguish themselves by focusing on different aspects of the nightmare. “The Andromeda Strain” concentrated on scientific plausibility; “Children of Men” struck notes of despair and near-religious salvation; a zombie flick is mostly interested in the gory results of infection. “It Comes at Night” wants to show how fear turns people ugly, inward and against each other. With its top-notch cast and a pervasive sense of dread courtesy of writer-director Shults, “It Comes at Night” makes for an intense viewing experience even if its story doesn’t have the full dramatic impact it promises.

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Like many horror movies, “It Comes at Night” suggests that its central family has somehow opened its door to evil. A prowler turns out to be another dad, Will (Christopher Abbott), who endures some initial brutality — Paul ties him to a tree for several days to watch for any tell-tale pustules of illness — but is then invited to live in the house. Will brings his wife, Kim (Riley Keough) and their little boy (Griffin Robert Faulkner), whose innocence brightens up the place. The new communal happiness fades when Will gets caught in a small lie and Travis’ pent-up libido becomes fixated on Kim.

These moments work to give us little shivers of anxiety, but they never develop into anything larger or more thematically significant. If Shults’ movie has a point to make, it’s that fear can rob even good people of their humanity. That’s not a new insight, but “It Comes at Night” delivers it with grim conviction.