Something serious is bugging Jay (Maika Monroe), center, in “It Follows,” but it’s tough to explain it to neighbor Greg (Daniel Zovatto) and her sister Kelly (Lili Sepe). (AP)

It Follows” is one of the scariest movies I’ve ever seen. It’s also one of the most beautiful.

Set in and around Detroit and alternating between eerily still suburban streets and the most decaying parts of that city’s downtown, the film — about a shape-shifting bogeyman plaguing a teenage girl — has the look and feel of a pleasant dream that is slowly, inexorably turning out to be a nightmare.

Subtly off-kilter production design (by Michael Perry) makes the time of the story hard to place: There are no cellphones here; television sets are all about 40 years out of date; yet one character runs around reading Dostoevsky’s “The Idiot” on a Kindle-like touchscreen set inside a handheld gadget resembling a make-up compact. Mike Gioulakis’s atmospheric cinematography, which calls to mind the enigmatic, meticulously art-directed photographs of Gregory Crewdson, belongs in an art gallery. And the story, written and directed by David Robert Mitchell (“The Myth of the American Sleepover”), gives off a perfume of such rich, intoxicating ambiguity that the viewer is constantly off balance.

What exactly is tormenting Jay (Maika Monroe) is just as difficult to pin down.

After a sexual tryst with a boy named Hugh (Jake Weary), high-schooler Jay learns that she has been infected, in a manner of speaking, by a relentless supernatural pursuer. Taking the form of a stranger, acquaintance or even a loved one — in seemingly random fashion — he, she (or, more aptly, it) simply comes after Jay, on foot, with malevolent intent. The only way to pass it on is by sleeping with someone else, who will then become the next victim, Hugh tells her (before unceremoniously dumping her).

It might be the world’s first venereal stalker. Walking, never rushing, the “it” of “It Follows” is something from which you can run, but never hide. Monroe heads up a fine ensemble cast of young, mostly little-known actors who ground the film in a kind of banal believability.

As a filmmaker, Mitchell isn’t afraid of genre cliches. The association of death with adolescent sexuality, for instance, is merely the most obvious of several allusions to horror conventions that the director plays with. But if his tropes are, at times, uninspired, he never relies on them as a crutch. There’s no attempt to explain Jay’s tormentor as, for example, a Babylonian deity that has been inadvertently summoned through an incantation found in an old book. And there are — most thankfully — exactly zero references to “feeding on fear.”

Rather, “It Follows” is content to let the mystery, and the terror, grow from an unknowable seed. We, along with Jay, can see this thing that no one else can, but there is no effort made to define it. That makes it all the more frightening.

And relentless. “It’s slow,” Hugh warns Jay, after introducing her to her new curse. “But it isn’t dumb.”

The same might be said for “It Follows.”

The film has a deliberate yet canny pace that matches the methodical languor of its villain. If there are parts where the action slackens, they’re never torpid or dull. The unease is always there, even if unseen.

In other words, “It Follows” sticks to you — yes, even outside of the theater — with a grim unshakability that is at once stylish, smart and deadly serious.

R. At area theaters. Contains sex and nudity, frightening and violent imagery, coarse language and teen smoking. 100 minutes.