The “Insidious” horror franchise, which began in 2010, has gotten so chronologically convoluted that newcomers might find the need to shuffle the installments around to find their bearings. Fortunately for those just coming to the series, getting up to speed would merely require them to watch “Insidious: Chapter 3” (2015) before taking in “Insidious: The Lost Key” (or “Chapter 4,” if you prefer), which is really a direct sequel to it. Then they can move on to the first “Insidious,” the action of which is immediately subsequent to that of “Key,” and to its 2013 sequel, “Insidious: Chapter 2.”

Or you could do the smart thing and just watch the original “Insidious” and stop. It’s a reasonably clever—and relatively gore-free—modern horror movie, even if it trailed off in the final act. The remaining movies represent the law of diminishing returns, and this “Key,” along with its immediate predecessor, proves the nadir—at least until another chapter shows up, because, while this installment represents the closing of the arc for the woman who’s become its central character, it still leaves the door open for continuations with a different, younger heroine in charge. (Given the history of horror franchises, of course, the appearance of a word like “Last” or “Final” in a subtitle is no guarantee that we’re actually at an end. In fact, it usually indicates the opposite.)

In any event, “The Last Key” brings full cycle the story of Elise Rainier (Lin Shaye), the elderly psychic whom the troubled Lamberts (Patrick Wilson and Rose Byrne) summoned in “Insidious” to help them exorcise the demon that had taken over their young son (Ty Simpkins). Rainier died at the close of that movie, and appeared only spectrally in the sequel. But she took center stage with “Chapter 3,” which backtracked to the first exorcism in which she was joined by her comic-relief aides Specs (Leigh Whannell) and Tucker (Angus Sampson).

In the aftermath of that mediocre frightfest, this film now gives Elise the full bio-treatment, beginning with a long prologue, beginning in 1953, showing her sad childhood in a New Mexico house she (first Ava Kolker, then Hana Hayes) shared with her loving mother Aubrey (Tessa Ferrer), her younger brother Christian (Pierce Pope, and then Thomas Robie) and her cruel father Gerald (Josh Stewart), a corrections officer at the nearby state prison. Gerald is obsessed with brutally punishing Elise for claiming to see ghosts, and during one stint in the house’s basement bomb shelter she unwittingly unlocks a door that releases a demon with keys for fingers (Javier Botet) that takes her mother’s life. She finally runs away, leaving Christopher alone with wrathful Gerald.

At that point the film switches to 2010, and Elise (Shayne) is approached by Garza (Kirk Acevedo), the new owner of her childhood home, to exorcise the malignant forces in it. So she returns to New Mexico with a mission. In the process she will also reconnect with her brother (now Bruce Davison), still bitter over her abandonment of him, and meet his daughters Imogen (Caitlin Gerard) and Melissa (Spencer Locke). In the process of her investigation she will discover the extent of the evil she unleashed so many years ago, and by travelling once more into what she calls “the Further,” will rescue a few endangered people, give some souls respite, and put that demon back where he belongs.

Looked back on in retrospect, the plot of “The Last Key” does pretty much parse (except for a thread involving Melissa, which seems utterly extraneous), but as directed by newcomer Adam Robitel it doesn’t make for a very scary movie. Much of it consists of Shane stumbling about the house in the dark of night, searching for spirits, and despite the efforts of Robitel, production designer Melanie Jones, cinematographer Toby Oliver, editor Timothy Alverson, the effects team and composer Joseph Bishara, those languid sequences don’t generate much suspense apart from the usual quota of sudden “gotcha!” moments accompanied by a sudden loud shriek on the soundtrack. (To his credit, Robitel does upend expectations at one point, setting up a shot that appears to be leading to an abrupt shock but then pulling back from it.) The result is more tedious than frightening.

And the comic relief moments offer none. The nerd act of Whannell and Tucker has gotten increasingly tired over the years, and their limp dialogue this time around—combined with their amateurish acting—is very hard to stomach. Their constantly inept efforts to hit on Elise’s nieces comes off as especially creepy, given the script’s revelations about what the demon influenced both Gerald and Garza to do.

Acting doesn’t mean much in this sort of fare, but apart from Whannell and Sampson, everyone in the supporting column carries off their duties decently enough (though veteran Davison, to tell the truth, looks rather lost). Wilson, Byrne and Simpkins appear briefly toward the close in an allusion to their already-completed storyline. The major player, though, is undoubtedly Shaye, for whom this series has provided an unexpected career capstone. At a few points here she seems a mite tentative (or simply frail)—which might be attributable to slack direction or editing—but overall she pulls off the nonsense capably enough. One can imagine her acting as a spectral mentor to a younger heroine in something like “Insidious 5: A New Beginning.”

“The Last Key” may satisfy the most undemanding fans of the franchise by fleshing out Elise Rainier’s backstory, but on its own it’s just a mediocre haunted house movie. By the time it ends you may find yourself nodding in agreement with Elise when she says, “I should never have come back here.”