In the desert outpost of Five Keys, New Mexico in 1953, the Rainier family lives so close to the federal penitentiary that all the lights in the house flicker from the surge of a nearby electric chair. While her little brother Christian greets the occasion with boyish enthusiasm ("You're on the Hades Express, mister!"), Elise quietly sketches a vision of the man in his final moments and recites certain facts about him, like how he chose a ribeye steak for his last meal and told the witnesses to "Go to hell!" before the executioner flipped the switch. Her father, a mean and abusive drunk, dismisses his daughter's extrasensory gifts as fantasy, but anyone familiar with the previous three entries in the Insidious horror franchise know otherwise. This is Elise Rainier's business.

As a conduit between the material world and the spirit world (known as "The Further"), Lin Shaye's Elise has become the center of the Insidious series, which started as a quick-and-dirty haunted house movie from director James Wan and writer Leigh Whannell, the team responsible for the Saw phenomenon. Much like Elise, Wan and Whannell were themselves acting as conduits, shrewdly bridging the retro-'80s horror of Poltergeist and Ghostbusters with the more aggressive, digitally enhanced shocks of contemporary studio horror. The sequels have edged more toward the Ghostbusters side of that equation, with Elise and her exceedingly goofy partners, played by Angus Sampson and Whannell, offering their spook-expelling services to those in need.

The fourth entry, Insidious: The Last Key, shows some inevitable signs of wear-and-tear, but that shift in perspective from the home-dwellers to the exterminators has distinguished it from typical haunted-house fare. Though it delivers the requisite stingers — albeit not as skillfully as past entries — there's something fundamentally whimsical about Elise and company sputtering along from one case to another like the Scooby Gang in their Mystery Machine. And Whannell, who has scripted all four of them, seems aware of it, too: In The Last Key, the Spectral Sightings crew travels in a custom-designed black Winnebago they picked up for $700.

In the series' chronology, The Last Key connects Insidious: Chapter 3, a prequel, with the events of the first film, though the pleasing tidiness of the plotting doesn't eliminate the possibility of a fifth entry. After starting with harrowing scenes from her childhood in 1953, the story opens in 2010 California with Elise getting a call from Ted Garza (Kirk Acevedo), the current tenant of the New Mexico home she had fled decades earlier. With Tucker (Sampson) and Specs (Whannell) working support, Elise discovers that all the malevolent old spirits are present again, marshaled by a creature with sharp metal keys for fingers. Those keys open the door into "The Further," which Elise must enter not only to sort out her client's problems, but to quiet the ghosts that haunt her soul, too. She also reunites with Christian (Bruce Davison), the brother she'd abandoned when she ran away from home.

The Last Key queasily engages with flesh-and-blood instances of horror and abuse, not just spectral ones, and the conclusion it draws about evil as an external force has the unfortunate effect of absolving real-life monsters of responsibility. The film is on steadier ground, however, when the Spectral Sightings team springs into action through various low-tech forms of ghost-busting, from amateur hypnosis sessions to crudely jury-rigged cameras and directional microphones. The hot-and-cold interplay between Shaye's earnest, determined seer and Sampson and Whannell's dopily enthusiastic nerds is by far the film's most appealing feature, because it pushes against the gothic self-seriousness that often smothers the genre.

Once the action shifts into "The Further," however, and director Adam Robitel has to conjure up a metaphysical hell on a budget, The Last Key starts to resemble a more typical dregs-of-January studio horror film. Four films into the series, the layout of the spirit realm has become too familiar, a soundstage of fog machines and grotesquerie that Robitel and Whannell haven't populated with any fresh shocks. If there's something strange in your neighborhood, the Insidious movies have answered the question of "Who you gonna call?" But there's only so many times you can hit redial.

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