Adding to cinema’s long list of hellish bachelor parties to which nobody in their right mind should accept an invitation, “It’s What’s Inside” gathers a large crowd of mostly estranged friends in a remote mansion where either no one can hear you scream, or no one much cares if they do. It’s an age-old setup for a body-countdown horror movie, and it’s to the credit of Greg Jardin‘s highly strung, busily plotted debut feature that it doesn’t unfold exactly as you’d expect. That’s down to a nifty high-concept premise — not wholly original, but more commonly used for purposes of comedy than horror — that the filmmakers are eager to keep a secret, which might be a challenge if this grabby, nasty Sundance Midnight premiere gathers the “Talk to Me”-level buzz it’s clearly targeting.

It begins, somewhat tellingly, with a failed exercise in roleplay. We open on Shelby (Brittany O’Grady), a cautious young woman, donning a wig and a vampish new persona in an attempt to get her her detached boyfriend Cyrus (James Morosini) to look at her differently — or, really, to look at her at all. Catching him instead mid-masturbation over laptop porn, she abandons the disguise, and they agree to defer a larger discussion about their sex life until later, after the weekend wedding celebrations of their friend Reuben (Devon Terrell). It’s not the first time in “It’s What’s Inside” that relationship fault lines will be exposed when characters casually adopt new identities, all in the interests of spicing things up; the film snuffles out the truths that emerge when everyone is masked, to steadily more hysterical effect.

Before the wedding comes a stag night of sorts, though with entertainment markedly different from the standard offering of strippers and lapdances. Shelby and Cyrus are among a small, unisex group of Reuben’s college pals summoned to his sprawling, creepy family manse for an evening of reminiscing and boozy horseplay away from the eyes of his fiancée Sophia (Aly Nordlie). It’s been eight years since graduation, and they haven’t all stayed as close as they might have done, though it’s a flaw of Jardin’s fairly diagrammatic script that it’s hard to imagine what might ever have bonded this rather random array of superficially drawn characters.

Among the guests are blonde glamazon Nikki (Alycia Debnam-Carey), who has built a vast online following as an influencer who mixes earnestly right-on activism with her acai-bowl snaps, and onetime golden boy Dennis (Gavin Leatherwood), now a tattooed try-hard who doesn’t do much of anything. Brooke (Reina Hardesty) and Maya (Nina Bloomgarden) are also in the mix, albeit somewhat anonymously, but conversation swirls around an eighth, unconfirmed invitee: Forbes (David Thompson), a gawky misfit whom no one has seen since he dropped out of college in controversial circumstances, though rumor has it he’s remade himself as some manner of Silicon Valley mogul. Sure enough, he turns up late to the party, carrying a chip on his shoulder and a briefcase in his hand — inside, his latest technological development, a kind of mind-warp game that he’s eager to test on his estranged friends.

Collectively, they do what anyone in the movies does when an enigmatic, slightly chilly figure turns up promising to show them a new reality, and what nobody would do otherwise: They drink up and dive in. The first act of “It’s What’s Inside” may not bear close scrutiny, but Jardin is in a hurry to get to the frenetic debauchery of the second, where — without straying into spoilers — the characters’ hitherto wary defenses are very swiftly broken down, lines are crossed and perspectives traded. Shelby and Cyrus’s already fragile relationship, in particular, is put through a sharply exposing form of group therapy, in which hidden motivations and desires float rudely to the surface.

Jardin conducts all this neon-lit social anarchy with energy and aplomb, sparing neither his characters nor his audience the essential cruelty of the premise, though spiking proceedings with the kind of shrill, fizzily caricatured comedy that drove Halina Reijn’s comparably constructed horror “Bodies Bodies Bodies” in 2022. The results are coldly diverting, with the plot continually ratcheting itself into higher degrees of panic and surprise, though potential for a darker, harder psychological payoff is missed — largely because these characters are so thin, it’s hard to care much for their vulnerabilities or to keep track of their evolving personae. Stabs of social-media satire — targeting both Nikki’s do-gooder wellness brand and Reuben’s exhaustively hashtagged wedding plans — are amusing but easy, glossing over the deeper human weaknesses underpinning them.

Still, Jardin approaches this story rather as the smirking, meddling Forbes seemingly enters the party: as an agent of chaos, out to mix and mess things up for the sheer hell of it. On these terms, “It’s What’s Inside” registers as a pretty effective genre calling-card: gleamingly shot and spikily cut, brashly acted by actors accepting of their role as pretty pawns in an elaborate narrative game of strategy. Studios seeking a zesty stylist for a malleable horror script would do well to take his number, though one hopes future projects will tease out this debut’s flickering hints at more tortured, subversive human interests.