If you’re just dying to know how psychic Elise Rainier (Lin Shaye) first met Tucker (Angus Sampson) and Specs (Leigh Whannell), her none-too-bright assistants in the first two installments of the horror franchise, “Insidious: Chapter 3” will tell you. Otherwise this prequel, penned and directed by Whannell, is nothing more than a threadbare possession tale gussied up with some haplessly mediocre shock effects. More tedious than frightening, it should certainly be the last in the series; but who knows, when the other Wan-Whannell series, “Saw,” lumbered on for seven movies, each worse than the last?

The plot has Rainier coming out of retirement after the events recorded in the flashback portion of “Insidious 2”—the haunting of young Josh Lambert (Garrett Ryan), briefly alluded to in stills here—to save Quinn Brenner (Stefanie Scott), a teen who, in trying to contact her recently-deceased mother Lily (Ele Keats), has conjured up a malevolent being from the netherworld, a gray apparition called The Man Who Can’t Breathe (Michael Reid MacKay), that aims to take over her soul. (Rainier’s opted out of the psychic business because she’s now being pursued by the murderous female spirit that’s central to the entire “Insidious” mythology. Also, her husband recently committed suicide, and she’s grieving for him.) Nonetheless, after conferring with her old helpmate Carl (Steve Coulter) and steeling herself for battle, Elise saves Quinn with the help of the girl’s father Sean (Dermot Mulroney, who looks understandably morose throughout), Tucker and Specs—Internet ghostbusters summoned into service by Quinn’s younger brother Alex (Tate Berney)—as well as the spirit of Lily. In the process she, Tucker and Specs forge an unlikely working relationship that circles back to the initial “Insidious.”

That first film, written by Whannell but directed by James Wan, was a superior example of its type, imaginative and cunningly staged until a final act that went into overdrive with effects and cheap shocks. The follow-up was a distinct letdown, crippled by a numbingly convoluted scenario with so much back story one had to take notes just to keep things straight.

By contrast this new film is pretty much just a rehash of the first, relying on the hoariest of genre tropes and proceeding like molasses on a warm day. It starts by putting Quinn in the same sort of disabled condition that little Dylan suffered in the initial movie—he was in a coma, she’s incapacitated with two broken legs—and then has her confronted by the spectral menace that haunts the apartment building where the Brenner family lives. The afterlife—which Rainier here dubs “the Further” for the first time and must enter in order to save the girl—is nothing more than the hallways of the high-rise shot in gloomy darkness (due to budgetary limitations, no doubt). The spirits she encounters there look like refugees from a Japanese horror movie, with the “half” of Quinn that’s been taken over by her nemesis crawling about like so many of the faceless female ghosts in the myriad clones of the “Grudge” pictures. The final confrontation is a poorly-choreographed bust, and the inevitable “gotcha” coda totally pointless. And within the confines of the basic plot, the script introduces a bevy of lesser characters—an elderly neighbor (Jeris Poindexter) and his addled wife (Phyllis Applegate), Quinn’s best friend (Hayley Kiyoko), a geeky kid with a crush on Quinn (Aston Moio) among them—whom it then simply drops with little explanation, making for even greater narrative chaos.

In short, “Insidious: Chapter 3” is a shambles. If it has any saving graces at all, they’re Shaye and a gratifying sense of restraint in the gore department. The series has finally given the veteran supporting actress her place in the sun, and though her work is hardly subtle, she makes the most of the opportunity to shine. Like the previous installments, moreover, the movie eschews the blood-and-guts approach of today’s explicit horror pictures, going instead for mild shock effects—shadowy figures glimpsed on the edge of frame or suddenly lurching into view (often from behind the characters), always accompanied by a burst of Joseph Bishara’s music. The emphasis on such old-fashioned scare tactics makes for a rather tame movie by contemporary genre standards, but at least one doesn’t have to dread being offended by close-up shots of steaming entrails, severed limbs or worse. The picture might not be scary or exciting, but at least it isn’t vile.

That’s faint praise, though, for a movie that you might appreciate for its unwillingness to turn your stomach, but that won’t raise many pleasurable goosebumps either.