Writer-director Scott Stewart, who’s made bad special effects extravaganzas (“Legion,” “Priest”) up to now, goes the original thriller route this time around. But to call “Dark Skies” original in any real sense would be stretching the word beyond endurance. The movie seems to be a compendium of bits and pieces of Stewart’s favorite pictures. It begins as a “Poltergeist” clone that eventually morphs into an alien-abduction story, complete with multiple nods to the “Paranormal Activity” franchise. Along the way it tosses in shards of a Spielbergian boy-coming-of-age flick and lots of pseudo-Hitchcockian scare moments. (Stewart manages to find room for a sequence that mimics “The Birds,” of all things.) Even the title is ripped off from a failed NBC sci-fi series from the mid-nineties. Since all the second-hand ingredients are delivered without much style or flair, the movie makes for a pretty insipid dish.

The suburban family that suffers through the script’s cascade of traumatic experiences are the Barretts. Dad Daniel (Josh Hamilton) is an out-of-work architect, increasingly desperate over finances; mom Lacy (Keri Russell) is a real-estate salesperson hard pressed to make a sale in the down economy. They have two sons: adolescent Jesse (Dakota Goyo), who’s feeling his first hormonal urges, and kid brother Sam (Kadan Rockett), who’s fascinated by the scary stories Jesse reads to him over their walkie-talkies after both have been tucked in for the night.

Trouble begins when the Barrett house is invaded by strange forces that mess up the kitchen or arrange cans and pots in tall geometric arrays. Since the cops are of no help, Daniel sets up—despite the family’s strained circumstances—an elaborate battery of video equipment to watch over the rooms at night. But since the monitors have a way of conking out at significant moments, that proves of marginal use.

Meanwhile Sam reports being accosted by nocturnal visitors he calls “the Sandmen” and begins sleepwalking, even in the daytime. Daniel and Lacy suffer blackouts in which they lose hours of time. Flocks of birds crash into the house. Sam’s body exhibits bruises that bring charges of child abuse. In between his halting approaches to neighbor girl Shelly (Annie Thurman) and his fraternizing with hell-raising older neighbor Kevin Ratner (L.J. Benet), Jesse suffers a seizure—and what’s with those geometric symbols the doctor finds on his skin? And as if all that weren’t enough, flickering street lights—which Stewart uses as a tired motif—suggest that something uncanny is going on.

In response to the unexplained circumstances, Lacy cruises the Internet for answers, leading her and Daniel to consult an alien-invasion expert named Pollard (J.K. Simmons), who just happens to live in the selfsame town, and who sadly informs them that they’ve been targeted as guinea pigs by “the Grays,” visitors from beyond that have long been experimenting on humankind. He says they’re likely to abduct the person whom they first contacted in the house. That leads mom and dad to alien-proof the place, arm themselves with guns and knives, and prepare to ward off the threat, which comes in the form of beings apparently adept at creating hallucinations to draw their quarry in. The denouement involves a twist that dredges up clues from earlier reels in the fashion of “The Usual Suspects,” followed by one final “Twilight Zone”-like moment, but by then the movie has run totally out of gas.

The cast is better than what one usually encounters in such fare. Russell actually seems to take the material seriously and goes the strenuous dramatic route with some success, and Simmons brings a convincingly world-weary sense to Pollard (perhaps reflecting the mood the audience might be in by that time). Hamilton, on the other hand, is blankly inexpressive and appears to convey callowness all too easily. Stewart doesn’t appear to have much skill directing the children: neither Goyo nor Rockett shows the naturalness one ideally wants in such roles, and the other youngsters in the film are equally ill-at-ease. Technically the movie betrays its modest budget at every turn, from the flat, undistinguished cinematography of David Boyd to the bland score by Joseph Bishara.

One shouldn’t be too hard on “Dark Skies.” Doing so might lead Stewart to return to the sort of grim special-effects fiascoes he’s made before, and you certainly don’t want to encourage that. But it’s one of those mediocre genre mash-ups that really has no reason to exist.