There’s a demon lurking in your smartphone—in addition to Mark Zuckerberg, that is. It’s the Satanic force behind the titular app in Justin Dec’s feeble little horror movie, which doesn’t even merit being watched on a phone’s postage-stamp-sized screen, let alone in a theatre.

The heroine of the piece is Quinn Harris (Elizabeth Lail), a nurse-in-training at a hospital in some nameless town, where she works with Dr. Sullivan (Peter Facinelli), who’s an obvious sexual predator, though neither she nor any of her co-workers are smart enough to recognize that. She’s kind to everyone, including Evan (Dillon Lane), a patient nervously awaiting surgery for injuries he suffered in a car crash.

His squeamishness arises from the fact that his phone is running an undeletable app, Countdown, which supposedly tells you the very moment you will die and, as the name says, counts down to it, second by second; and it tells him he will die during the operation. He doesn’t, actually, but that’s because he violates the user’s agreement that people robotically accept even without being required to prove they’re not robots—just as his girlfriend Courtney (Anne Winters) did when she refused to let him drive her home when he was drunk. Trying to avoid your fate, you see, automatically results in some scary apparition showing up to do you in anyway: that’s the deal.

Lots of the hospital staff, including Quinn, have foolishly loaded the app onto their phones as a lark (so will her younger sister Jordan, played by Talitha Bateman), but after Evan’s death she grows concerned about the fact that it’s predicting her death is right around the corner. She’ll find a comrade-in-arms in Matt Monroe (Jordan Callaway), who’s in a similar fix. Together they track down the inevitable Catholic priest (P.J. Byrne) who’s knowledgeable in demonology, though in this case the guy is a gee-whiz supernatural freak rather than an aged exorcist, and his reading of the coding (in Latin, no less) suggests the whole thing was composed by a soul-seeking demon and the only way to break the curse is for somebody to die too early or too late and thereby short-circuit the system.

How this all resolves itself won’t be revealed here; suffice it to say the main plot trajectory, though intended to be both scary and funny, is neither, while the subplots—especially that involving sleazy Sullivan—amount to irritations. To add insult to injury, not only does the resolution include one of those phony death-followed-by-resurrection scenes, but it’s followed by not one but two codas, a supposedly humorous bit featuring an inconsequential secondary character (Tom Segura) and a cemetery scene that might not copy the famous “Carrie” denouement but does something even more frightening: it paves the way for a sequel.

The craft contributions here are about par for this sort of horror potboiler, and so is the acting, with Calloway giving the most natural, unforced performance; Lail makes a pretty colorless heroine, and Byrne is ridiculously over-the-top as the priest. Charlie McDermott, late of “The Middle,” one of the best recent TV sitcoms, shows up in a couple of scenes as a hospital orderly; hey, Axl!

For those who loathe cellphones—especially when used in movie theatres—“Countdown” will at least afford an opportunity to observe those addicted to them get their just deserts. But that’s not a sufficient reason to recommend this remarkably dumb, fright-free Halloween trick.