The law of diminishing returns, which had already begun to operate in the last third of 2011’s “Insidious,” resumes in full force in James Wan’s misguided sequel, which gets tied up in narrative knots trying to come up with an explanation for what’s going on in the haunted Lambert household. The first “Insidious” took a wrong turn in 2011, and this follow-up is forced to go along with it. The result is that “Insidious 2” is not only complicated but silly, and the way the plot is worked out is pretty much a structural disaster.

After a prologue that takes us back to 1986 and shows hubby Josh (Patrick Wilson) as a young boy (Garrett Ryan) tormented by a spectral woman—just as his son Dalton (Ty Simpkins) has now been—but being freed from her control through hypnosis at the hands of young medium Elise Rainier (Lindsay Seim). Then the script switches to the immediate aftermath of the first movie, with the older Elise (Lin Shaye) murdered and the Lambert family—Josh, wife Renai (Rose Byrne), reawakened son Dalton, his brother Foster (Andrew Astor) and toddler Kali (Brynn and Madison Bowie)—taking refuge in the house of Josh’s mother Lorraine (Barbara Hershey). But there’s no sanctuary from malignant spirits, of course, and soon a ghostly woman (Danielle Bisutti) makes a threatening appearance.

That leads to a call to the late Elise’s duo of slapstick assistants, Specs (Leigh Whannell) and Tucker (Angus Sampson), and they in turn bring in Elise’s old ally in the ghost-hunting business, Carl (Steve Coulter), who as a young man (Hank Harris) worked with her on the case back in ’86. His efforts to contact Elise lead them, and Lorraine, to the derelict house of one Parker Crane (Tom Fitzpatrick), who, while on his deathbed in a hospital where Lorraine worked, once terrorized young Josh. It turns out that the man, who had mommy issues that we see in flashback, had a part in a murder spree years ago, which turns out to be the key to Dalton’s recent haunting.

Working all this out requires more episodes of possession of the bodies of the living by the spirits of the dead, lots of chases and assaults that cut from one group of characters to another with bewildering speed, the reappearance of Elise’s benign soul, and the emergence of what appears to be a small army of murdered brides. With each twist of the plot, things grow more and more convoluted and absurd—there’s even one scene that’s a bizarre allusion to “Mommy Dearest”—and when one of the main characters remarks on the pain he’s feeling, you can’t help mutter snidely that it’s nothing compared to what the audience is enduring.

One has to sympathize with Wilson, a bland actor who’s here faced with the task of breathing life into two versions of his character when he can barely cope with one. He does get an opportunity to fight himself, but one wonders whether he welcomed it. Byrne has the misfortune of playing a character who’s almost constantly being threatened, which reaches a miserable nadir in the final reel, when she’s being repeatedly punched and strangled on the kitchen floor. Shaye once more goes the tongue-in-cheek route with some success, and Whannell and Sampson do their Laurel and Hardy bit to modest effect, but though Hershey maintains her dignity throughout, it’s sad to see this fine actress reduced to such stuff. The youngsters are all adequate, but the adults in lesser roles chew the scenery mercilessly (particularly Bisutti and Fitzpatrick), though to be fair they’re just doing what Wan asks them to.

The director also shows his propensity for bits of business that are becoming increasingly tiresome—menacing dolls, toys that come to life, radios that turn themselves on, and so on—not to mention slamming doors, accompanied by loud musical shrieks on the soundtrack (courtesy of composer Joseph Bishara). And though the production crew manages atmospheric visuals on an obviously low budget and cinematographer John Leonetti provides moody widescreen images, nothing in the picture delivers the jolts that the first “Insidious,” or Wan’s more recent “The Conjuring,” did.

Speaking of which, this sequel should prompt concerns about what a possible “Conjuring 2” might bring. One is probably inevitable anyway, though; after all, the tacked-on prologue here suggests that the “Insidious” franchise could become another “Saw” in longevity. That isn’t a pleasant thought.