The publicity for this third installment in the faux-found-video-footage haunted house series promises answers to why the phenomena depicted in the two first pictures were happening; and “Paranormal Activity 3” does provide them. Unfortunately, they not only prove pretty humdrum (not to mention much too complicated for the outcome they’re apparently aiming at), but make the occurrences shown in the earlier films—which really come later chronologically—inexplicable. And the look of the film lacks the grit of its predecessors, rendering its whole “accidental” premise awfully feeble.

To dispense with the narrative first: this is a prequel supposedly dating from 1988, when the two sisters (played by Katie Featherstone and Sprague Grayden) whose California homes were invaded in the first two films were young kids living with their mother Julie (Lauren Bittner), and her new significant other Dennis (Chris Smith). (Dennis is either her second husband or a live-in boyfriend—it’s never entirely clear which.) Dennis happens to be a videographer with a business doing wedding photos, and so it’s natural that when things start going bump in the night in their house, he sets up batteries of video cameras to record the various rooms, and then apparently has time to scrutinize each frame of the myriad hours of resultant footage the next day. (It’s amazing, given that schedule, that we see him actually going to bed each night!)

It’s the same notion that fueled the preceding pictures, but the supposedly “raw” footage shown here is much less ragged and ad-hoc than that in them—less primitive, although it supposedly dates from two decades earlier. One could argue that Dennis was a professional, so his equipment was state-of-the-art and his set-ups more skilled. But how then to explain his “invention” of a rotating single camera, mounted on the base of a spinning fan, that can cover two rooms at once by shifting from one to the other? Yes, it allows the real filmmakers here (Henry Joost and Ariel Schulman, who made the obvious mockumentary “Catfish”) to set up some cool frights by shifting perspective just as a “gotcha” moment is about to occur—the most notable being a sequence involving a babysitter and a sheet that looks like it’s covering one of the girls. But it’s just a cheap directorial trick calculated to play on audience reactions rather than something Dennis would have done in his circumstances. If you really wanted to cover both rooms, why not set up two cameras, so you wouldn’t miss stuff?

Of course one could also criticize “Paranormal Activity 3” for its pedestrian acting and banal dialogue. But an even more fundamental problem is that the scenario simply doesn’t make much sense when the explanation for what’s happening is tacked on at the close. And since this review is now going to discuss that, it’s important to note that if a reader wants to preserve the picture’s surprise for a later date, he should stop reading here. SPOILERS ARE COMING.

“Paranormal Activity 3” ends up at the house of Julie’s mother (Hallie Foote), where she, Dennis and the girls have fled after the goings-on in their house have become intolerable. It turns out that grandma is the head of a witch’s coven that has presumably conjured up the spirits bedeviling the family and have apparently lured Dennis to the meeting-place to be done away with. (Julie appears to be complicit in the whole business, since she utterly refuses to entertain any evidence of supernatural stuff in her house.) But if that’s the purpose behind all the mayhem, why not just have simplified things by inviting the poor fellow over for a visit?

And then there are the girls. Is their involvement with the malignant spirit the younger of them accepts as her “imaginary friend” Toby accidental, or intended as some sort of initiation into the family business of witchcraft? Or are they singled out as sacrificial lambs? If the latter, why? And why have they forgotten so much of what happened to them as children by the time that they’re grown, in the first pictures? “Paranormal Activity 3” may offer answers to what’s been happening in this family’s unhappy homes, but to at least this viewer they’re inadequate ones.

To be sure, the movie is superior to most of the horror flicks out there nowadays in that the gore quotient is low and it depends for its shocks on techniques from a simpler, pre-torture porn times; a few of them work. And it adds some humorous touches, especially involving the babysitter and Dennis’ goofy business partner (Dustin Ingram). But like the third installment of virtually every franchise you can name, especially in the horror field, it pushes up against the law of diminishing returns. The “Paranormal Activity” series is basically a one-trick pony that’s played its stale sleight-of-hand once too often, and in trying to offer a quasi-logical explanation for the ghostly goings-on it comes a-cropper.