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The INSIDIOUS movies are easily the most idiosyncratic (successful) horror franchise of the moment, built out of elements like recurring characters, signature visuals, mythology and a unique internal logic that the rest of the genre has largely abandoned in favor of chasing grimy grit-gore (HOSTEL and MARTRS, but more so their lesser imitators) or cheapjack trickery (PARANORMAL ACTIVITY etc). That's what helps it stand out in a field that otherwise seems to be chasing forgettable as an ideal, but it can also be a trap pointing to diminishing returns: Eventually the 80s slashers with their iconic masks and signature weapons (particularly Freddy Krueger, whose as close to a direct ancestor as INSIDIOUS has) ceased to be scary through all the recognizability.
To be fair, INSIDIOUS stock in trade is a fairly unique brew: Small-space haunting/possession stories featuring violently-proactive "rule-breaking" specters (who manage to still be legitimately scary while being designed in an overly-specific "Halloween spook-house" style that shouldn't work but does) and visits to "the other side" ("The Further" in INSIDIOUS-speak) imagined through a weird mix of new-school FX and low-tech settings - usually just an actor holding a single light-source in an empty space full of dry-ice fog. That's a pretty damn unique stamp, and one that can probably sustain the series for a while longer, but CHAPTER 3 shows signs that the limit is significantly lower than the sky.
Let's be clear: The original INSIDIOUS was an out-of-nowhere masterpiece as far as I was concerned, and the sequel was an impressive effort boosted by one killer Big Idea (since "dead worlds" are outside time and space, why not add time travel to the bag of tricks?) but kneecapped by a baffling revival of one of horror's most unpleasant "things were different back then" character tropes. CHAPTER 3 brings nothing like that to the table, thankfully, but the innovation has been dialed back as well - it's easily the most conventional of the series, and while that doesn't have to be a deal-breaker it means the constant sense of "Wow! I can't believe they pulled that off!" that very much defined the prior installments (for me, at least) isn't very present.
This is technically a "prequel," but mainly for the purposes of keeping hardworking character-actress Lin Shaye around as the (still living version of) medium Elise. It's billed as an "origin" story, but really it's just a new, unrelated haunting yarn executed "Insidious-style" and with a quickie explanation for how Elise first hooked up with her younger, geekier colleagues from the previous two installments tacked on for Act 3. Nothing wrong with that, but I'd be lying if I said I wasn't more interested in the "ghost versus ghost" angle that the ending of Part II seemed to be pitching.
The story this time: A stressed-out teenage girl has been trying to contact the spirit of her recently-deceased mother, but has instead earned the attention of the malignant soul-gobbling spirit (an emaciated old man in an oxygen mask who leaves slimy black footprints everywhere - a servicable but non-classic heavy) haunting her apartment complex. The creature arranges for her to be injured and bedridden, making the torment and possession easier, and necessitating Elise (who at this point had retired because she's depressed at the suicide of her husband and fears meeting the lethal Bride in Black ghost again) getting back into the field to stop it.
The counter-intuitive non-spectacle of Shaye's unassuming heroine as a discount Doctor Strange is still the franchise's most amusing conceit, and the rest of the film fits together well enough, but there's a sameness setting in that's more pronounced now that you can't blame it on other returning actors of plot details. Co-creator and co-star Leigh Whannell takes over the directing duties and acquits himself ably in terms of keeping the style consistent, but what perhaps should've been a perfect place to start expanding the formula plays it pretty safe - if you know the "beats" of the series thus far (cluttered room = monster hiding motionless in plain sight, false/false/REAL rhythm for most scares, if one of those pale blue ghost-faces is hanging out JUST visibly enough in the frame something bigger is going to lunge out from another angle while you're busy squinting, etc) you're probably not going to jump very far back.
The finale, at least, is interestingly realized after all the work is done getting there: Nothing earth-shattering, but after decades of seeing variations on the seance/exorcism-to-reclaim-a-stolen-soul routine it's nice to see a new-ish twist on the proceedings (the Big Bad keeps a half-formed homonculus representing 1/2 of his target's soul like a pet, and it "fills out" as he claims more and more) that eventually blows up into a memory-totem/emotional-appeal/divine-intervention whirl that wouldn't have been out of place in a mid-80s Amblin production. I will say, though, that it's getting to the point where INSIDIOUS needs to start locking down exactly what meaningfully separates souls/ghosts/demons/remnants/etc in its mythos; since right now they appear to function interchangably.
These things tend to keep going for as long as they'll turn a profit, and having now used two secondary-endings to tease the return of a pivotal character one can only assume that's grist for the mill in CHAPTER 4. All well and good... except that probably means monster-backstory and the last time the series bothered to try that the results were pretty wonky. Then again, the FAST & FURIOUS movies took 5 to 6 tries to start getting good, so the "rules" of franchise-rot seem to have changed recently - but those are going to still have new ways to crash cars long after INSIDIOUS has run out of dark corners to stick ghosts in.
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