"If you call out to one of the dead," cautions retired psychic Elise Rainier (Lin Shaye), "all of them can hear you." It is a warning that teenager Quinn Brenner (Stefanie Scott) fails to heed after showing up on Elise's doorstep with a plea to help contact her deceased mother. In exactly the next scene, she is lying in bed, the chains from a dangling ceiling lamp casting eerie shadows on her wall, when she thinks nothing of asking out loud if her mom is there. She isn't, but something else, it turns out, is. A prequel to 2011's super-scary funhouse ride "Insidious" and 2013's plodding "Insidious: Chapter 2," "Insidious: Chapter 3" has bursts of jittery inspiration and a first half that pays surprising attention to its characters even in the moments when specters aren't hanging from ceilings or hiding in wait behind curtains. Leigh Whannell (making his directorial debut, taking over for James Wan) displays a stylistic precision as a filmmaker, but is let down by his own screenplay, throwing in too many extraneous characters that he doesn't know what to do with and going slack during a syrupy denouement that loses grasp of its frightful hold.

A few years prior to the Lambert hauntings portrayed in the earlier installments, 17-year-old Quinn is struggling to make do following the painful loss of mom Lillith (Ele Keats). Living in a fourth-floor apartment with electrician dad Sean (Dermot Mulroney) and younger brother Alex (Tate Berney), she is torn between choosing to remain at home to help raise her sibling and leaping at an audition for a prestigious NY-based theatre school. With so many changes forthcoming, she desperately wants to know that her mom is looking down on her. What she gets in return isn't quite so loving. Freaky visitations begin to permeate her everyday life just before she is violently hit by a car. Returning home weeks later with two broken legs, Quinn quickly discovers that her convalescence is going to be anything but peaceful. A malignant, otherworldly parasite has latched itself onto her, gaining strength as it gradually pulls her into a dark, ghostly alternate plane that Elaine tells the family is called The Further.

Most of the goosebump-inducing segments in "Insidious: Chapter 3" are the ones that rely on restraint, suggestion and a giant heap of old-fashioned dread. They are also, as it happens, largely relegated to the first hour. Director Leigh Whannell takes his time at the onset to burrow beneath the surface of Quinn. She is an exceptionally well-developed protagonist, and her scenes as she grapples with plans for her future and talks to Elaine about why she has sought her out are unafraid to take their time and live in Quinn's reality before the more overt horror is unleashed. Her initial sightings happen outside of her home—she spots a silhouetted figure waving from the wings of a lonesome theatre, and not long after sees an old man wearing a breathing mask standing in the middle of the street, again beckoning to her—and they are just strange enough to disconcert in a big way. Later, there is an ingenious, seat-jolting bait-and-switch as Dean and Quinn successively look out an open window, while the image of a person walking across a room on broken legs that haven't yet healed excels with expertly vivid sound effects.

All of this is effective, indeed, but then it suddenly isn't as the ploys for scares become progressively repetitive and familiar. Supporting participants in the story, including Quinn's best friend (Hayley Kiyoko) and next-door neighbor (Ashton Moio), disappear entirely. Brother Alex is forgotten about for long stretches and then conveniently swept aside by the script. Even Quinn herself practically becomes a supporting player, the eloquent shading of her character dimming as the film reveals she has merely been a plot device in service of a different story: that of Elise's self-fulfilling journey back to doing her life's work, and the circumstances that led to her partnering with amateur ghost hunters Tucker (Angus Sampson) and Specs (Leigh Whannell). With the exception of an encounter Elise has in The Further with her late husband (Adrian Sparks), the third act falls decidedly flat as Whannell's bag of tricks prove rather unimaginative. The original "Insidious" depicted a similar harrowing climactic trip into the land of the dead with a great deal more twisted punch and conceptual diversity.

"Insidious: Chapter 3" always seems to be on the verge of reaching the next level but settles too often for the same old thing. Dermot Mulroney (2013's "August: Osage County") is given little to work with as widowed father Sean, but Stefanie Scott (2011's "No Strings Attached") is instantly engaging and sympathetic as Quinn. Leigh Whannell (2007's "Death Sentence") and a mohawked Angus Sampson (2015's "Mad Max: Fury Road") continue to be rather unctuous as Specs and Tucker, comic relief that simply isn't as amusing as either of them think they are. That leaves the invaluable Lin Shaye (2014's "Ouija"), returning as Elise, to steal the show. This is Shaye's most substantial turn yet within the franchise, and the layers of melancholy, fear and ultimate strength she exhibits as she gives herself over to a larger purpose are beautifully performed. Sadly, as compelling as a handful of spooky set-pieces are, the film does not consistently hit its target and loses its way by never quite figuring out which character and narrative thread should be the focal point. A welcome last-minute return of a memorable demon from the first film is also squandered, missing the one key detail that would have put this particular socko ending over the top: Tiny Tim's intensely creepy, missing-in-action "Tiptoe Through the Tulips." The "Insidious" series kicked off with a terrifying bang in 2011 and since then hasn't been able to match its heightened thrills and inventiveness. The law of diminishing returns regrettably continues with "Insidious: Chapter 3."