The original 2009 “Paranormal Activity”—the “Blair Witch” of its time—has been sequeled and copied so often by now that the latest installment in the franchise, “The Ghost Dimension,” would probably have worked better as a parody, so long as the Wayan brothers weren’t involved. Unfortunately, it’s being played straight, and the result is a deadly bore. The hapless priest who’s an obligatory presence certainly gets it right when, after opining that an exorcism would be useless, says “What we need is an extermination,” because the series has by now become the cinematic equivalent of an infestation that really needs to be wiped out.

The four installments preceding this one (not counting last year’s one-off “The Marked Ones”) have constructed, in shards, a fractured narrative involving a demon that haunts houses and targets those unlucky enough to live in them. But they’ve left important elements of the story out, or fudged them. This, purportedly the final picture in the series, tries to draw the fragments together into a complete explanation, but darned if the denouement doesn’t turn out to be both prosaic and irritatingly inconclusive.

But that’s only the last failing in a movie that for the most part just plows over the same well-worn ground as the earlier ones. Cheerful couple Ryan and Emily Fleege (Chris J. Murray and Brit Shaw) move with their little girl Leila (Ivy George) into a house in Santa Rosa that seemed like a real bargain. They’re joined by a couple of guests—Ryan’s brother Mike (Dan Gill), still nursing a broken heart after a bad breakup, and Emily’s chum Skyler (Olivia Taylor Dudley). It’s just before Christmas, and all are ready to celebrate.

But among the former occupants’ stuff Ryan finds a box of junk that includes an old video camera and a collection of family tapes. He gets the camera operating again, but finds that while he’s looking through the viewfinder it reveals strange, bubbly bits of energy in the house that can’t be seen with the naked eye. And the tapes show two girls being inducted into witchcraft by a strange man; these are obviously the younger Katie and Kristi from the previous films, who were trained by their coven-leading grandmother Lois. The house must be a rebuilt version of her place.

At the same time Leila begins acting strangely, exhibiting a predilection for creepy symbols and talking to an invisible friend named Toby, a name familiar to fans of the PA franchise, and poltergeist-like phenomena begin to occur throughout the house. So the Fleeges set up video cameras to capture the phenomena while protecting Leila. They even go so far as to call in that priest, Father Todd (Michael Kravic), whom they think might be able to help. But he explains that simply moving won’t solve anything, since demons follow their victims. And it’s pointed out that Leila, born 6/6/05, represents the devil’s number 666, since 2005 was the sixth year of the decade.

So what’s going on? It seems that everything the poor Fleeges are experiencing goes back to Grandma Lois’ arrangement with Toby, which involved providing him with the children he needs to assume human form (though why he should want to do that is never made clear). And with the acquisition of Leila—who’s seduced by a hallway that appears through the wall above her bed—he’ll have all the children he needs.

Most of what happens in “The Ghost Dimension” is overly reminiscent of the previous pictures, but though the bad acting and slovenly editing are constants, a couple of differences are apparent here. One is that first-time director Gregory Plotkin and his cinematographer John W. Rutland lean less on the series’ signature gimmick—those static camera shots that show manifestations occurring around the frame; here one gets more often the frantic camera-follows-action technique that’s in line with “Blair Witch.” (It doesn’t help.) The other is that for the first time the 3D format is employed. It adds a bit to the earlier instances of supernatural doings in the house, but it’s not until the big finale that the makers go for broke with it. Unfortunately, they wind up with swirling apparitions that are no more frightening than what you’d find in a chintzy carnival attraction. Otherwise the scares are meant to come from sudden bursts of paranormal exhibitionism that appear throughout the picture, always accompanied by a crack of sound. These interruptions are by now so commonplace that they don’t cause even a flinch, let alone a gasp.

Even the most dedicated fan will find this a soporific conclusion to a series that has been careening downhill since the first installment and now reaches a dead end in more ways than one.