The second sequel to the 2010 horror hit Insidious is actually a prequel, but it's a stretch to call it a freestanding scare story — or a "story" at all. Structurally, this movie has less in common with a classic haunted-house film than with a high-tech theme-park scarehouse. The shocks and jolts keep coming, and some are effective. But Leigh Whannell, the series cowriter who has taken over as director from James Wan (Furious 7), ties those scares to a plot so groaningly familiar, lopsided and poorly paced that it's impossible to care.

Gone is the ill-fated Lambert family, who starred in the previous two installments. The initial focal point of Chapter 3, set several years earlier, is Elise Rainier (Lin Shaye), the psychic who attempted to rescue two generations of Lamberts from demonic entities.

It's fun to watch the seventysomething character actor sink her teeth into a leading role, thin as this one is. Elise spends the film's entire first half waffling about coming to the aid of a dewy-eyed teenager named Quinn (Stefanie Scott), who thinks she's receiving cryptic messages from her dead mother. Elise quickly realizes a demon is actually buzzing the girl, but she hesitates to take action, tormented by a prophecy of her own future.

The opening conversation between the two women encapsulates the film's flaws. It drags on uneventfully. It's studded with awful dialogue. (Quinn on why her mom's cancer metastasized: "It's like it wanted to be with her just as much as everybody else always did.") If the scene nonetheless sends a few chills up spines, the credit belongs to lighting and production design. Jennifer Spence, who did the latter, deserves recognition for filling the film with unsettlingly cluttered or dilapidated interiors, places that look like they should be haunted.

A creepy neo-gothic apartment building is all we have to entertain us once the film's focus moves to Quinn and her dad (Dermot Mulroney) and brother (Tate Berney) — three actors in search of a single line of unclichéd dialogue or a believably scared facial expression. Perhaps Whannell hoped to rope in the Twilight audience with the scenes of Quinn's day-to-day life, but it's hard to imagine anyone connecting with Scott's one-note performance as a clueless ingenue. The creature who watches over her sleep isn't so sparkly, and her fate as possession fodder is foreordained.

The film picks up steam once Elise finally returns from psychic retirement. There are several well-timed jump scares, and the expeditions into the Further — the series' dark alternate dimension — showcase genuinely disturbing images.

But those images lack emotional impact when they're used to deck out a plot of dinner-theater flimsiness. The great haunted-house films inflict terror on characters that are already haunted by personal demons, blurring the line between troubled psyche and the supernatural. (Last year's The Babadook, currently available on Netflix, is a harrowing example.)

But Elise's personal conflicts simply don't have enough depth to resonate with the horrors she witnesses. Toward the film's end, she asks a fellow psychic, "Do you think we ever really help people?" It's an inside joke for the viewer, who knows that mediums in horror movies rarely ever do anything besides eat their own words. Like their scientist brethren, they represent a foolish attempt to master the unmasterable, because the supernatural entities of modern horror cinema observe just one rule: Keep the audience freaked out.

Unlike the campier — but more fun — Insidious: Chapter 2, Chapter 3 strives to observe that single rule religiously. Viewers who can ignore the characters and storyline, as one might at that theme-park attraction, will find the movie makes them jump a few times. But the kind of dread that gets under your skin — the insidious kind, one might say — has left the building.