Elise Rainier (Lin Shaye), psychic and conduit to the other side, has just awoke from a nightmare that is anything but. As a child (Ava Kolker), she witnessed her mother's (Tessa Ferrer) supernaturally induced murder, then was left to be raised by a monster of a different kind, her abusive, disbelieving father Gerald (Josh Stewart). Elise escaped his clutches as a teenager, but now, roughly fifty years later, she has been called back to her haunted childhood house in Five Keys, New Mexico, to help frightened current owner Ted Garza (Kirk Acevedo). Forced to unpack her own lingering traumas, she will ultimately discover even darker family secrets she never knew existed.

A supporting player introduced—and fatefully killed—in James Wan's enduringly freaky 2011 sleeper "Insidious," Lin Shaye's Elise proved to be such a standout that series-long screenwriter Leigh Whannell (2015's "Cooties") couldn't dare part with her for his lesser but still occasionally effective sequels, 2013's "Insidious: Chapter 2" (where she appeared as a spirit who guides the beleaguered Lambert family into the light) and 2015's "Insidious: Chapter 3" (a prequel where she was still very much alive). For fourth and hopefully final installment "Insidious: The Last Key," Lin Shaye (2016's "Ouija: Origin of Evil") finally becomes the focal point of a plot which fits chronologically between the third and first films while also providing welcome backstory to her upbringing and the early years of her parapsychic abilities. Director Adam Robitel (2014's unsettling, overlooked gem "The Taking of Deborah Logan") knows a thing or two about mercilessly milking tension out of jack-in-the-box set-pieces, but the banal script he is working from is nowhere near equal to his craftsmanship.

The first half is the picture's most involving segment, beginning with a nail-biting 1953 prologue and keeping the creep-factor high through the 2010-set scenes where Elise returns to her old house and begins her investigation. The setting, for one, is a triumph of production design, Elise's childhood abode sitting atop a hill next to a penitentiary where inmates are regularly sent to the electric chair. The interpersonal demons she must face are given enough time at the onset to ring true, including a poignant reunion she shares with the bitter younger brother, Christian (Bruce Davison), she abandoned all those decades ago.

Before the first hour is up, however, the repetitive nature of the narrative wears out its welcome. So much of the running time is devoted to Elise and her "Spectral Sightings" sidekicks Specs (Leigh Whannell) and Tucker (Angus Sampson) wandering around in the dark waiting for phantoms to pop out that it threatens to lose all of its potency. With the exception of one expertly designed sequence where Elise stumbles upon a stack of suitcases in a tunnel, the scares eventually dry up and their supposed payoffs become more plodding than thrilling. Caitlin Gerard (2012's "Magic Mike") and Spencer Locke (2012's "Detention") show promise early on as Christian's grown daughters Imogen and Melissa, but their characters are so glaringly underwritten they do not get the chance to come into their own. The film simply becomes too preoccupied with its next jump tactic.

Beyond the genre-ready skill director Adam Robitel exhibits, the real highlight of the film is Lin Shaye. To be the centerpiece of her own big-screen franchise at the age of 74 is uniquely special, and she is such a sincere, likable and nuanced actor it is a pleasure to watch her continue to build and mold the many facets Elise has to offer. By now, she is a three-dimensional protagonist whose life audiences should feel as if they intimately know. It is a notable letdown, then, that Elise is rendered little more than a pawn by the second hour, the observant humanistic eye of what has come before shoved aside for the same basic wait-and-pounce setup repeated ad nauseam until the end credits arrive.

There is a certain pathos in the quietly telling circular nature of its last minute, but by then "Insidious: The Last Key" has become saturated in a been-there-done-that aura. The squandering of the series' scariest established demons—the Bride in Black, who has supposedly followed Elise for most of her life, is entirely absent for no good reason—and the choice to once again not put to chilling use Tiny Tim's "Tiptoe Through the Tulips" (such an indelible part of the first two movies) is enough to leave one scratching his or her head. Somehow, the film seems unfinished without these elements. Tying something of a bow on what ought to be left a quadrilogy, "Insidious: The Last Key" joins "Insidious: Chapter 2" and "Insidious: Chapter 3" as faint shadows of the superior original. If "Insidious" continues to stand as a gem of the horror genre with a spine-tingling power which refuses to subside no matter how many times it is seen, its successors have never quite been able to capture that same macabre magic. One thing continues to be certain, though: in embodying the one-of-a-kind Elise Rainier, Lin Shaye gives it everything she's got.