Sex has long been a punishable offense in slasher movies but “X" cleverly flips the script. The movie, which opens in theaters Friday, juggles all the expected tropes but shuffles them around to make not a whole-cloth original but a patchwork homage that turns tired formulas new — quite appropriately since “X” is ultimately a gory struggle between young and old.

The first thing you notice about “X" is its command of atmosphere and camera movement. It opens in boxy academy ratio but pulls out to widescreen — an early hint that the film will conjure a ‘70s spirit in a way that’s highly conscious of the movie legacy its working in. “The Texas Chainsaw Massacre” is the most obvious touchstone here, but there are references throughout to films like “Psycho” and “The Shining.” Yet while a whole host of movies have eagerly cribbed from those films and others that “X” alludes to, it's striking how much the tensions and palpably human characters of “X” feel realistically grounded and viscerally their own. It's the difference between a glossy knockoff and the genuine article.

In a Dodge van marked “Plowing Service" are a group of amateurs who think they can make it big with their first porn film. Their chipper, confident leader and executive producer Wayne (Martin Henderson) is a cowboy who believes his girlfriend Maxine (Mia Goth) has “that x factor” to make her a star. She, with turquoise eyeshadow and a cocaine habit, agrees. Just as eager is Bobby-Lynne (Brittany Snow) and her boyfriend Jackson (Scott Mescudi, a.k.a. Kid Cudi), a laid-back Vietnam veteran. Directing and camera operating is RJ (Owen Campbell) who has brought his girlfriend Lorraine (Jenna Ortega) to hold the boom. She's slightly shocked to discover that they're making “smut,” as she says. RJ, explaining his artistic ambitions to prove that a dirty movie can be a good one, asks her when she became “such a prude.”

Right about then, in your average slasher, you might be thinking Lorraine, in her noble piety, is sure to outlast them all. I won't divulge anything in terms of murder order but the mayhem in “X” is tipped off not by lasciviousness but by fear of sexuality. Once they start making the movie, Lorraine is moved by the joyful experience, and wants to join in, herself. Now, it's RJ who can't handle his girlfriend's desires. Repression, not lust, is more likely to get you killed in “X."

Now about that older couple. Their names are Howard (Stephen Ure) and Pearl (Goth, again, unrecognizable under prosthetics). Pearl, we learn in surprisingly tender scenes with Maxine, still has yearnings despite her decrepit appearance. (Pearl's makeup seems modeled after the mother in “Psycho.”) Once a great beauty who could make her husband do anything she wanted, he's now too old for lovemaking. Those feelings of inadequacy and frustration, for the kids out back making a porno, spell trouble.

“X” loses some of its grip once the killings start. It would have been more powerful if Pearl was played by a genuinely older actor. Having Goth play the two characters contributes to our sense that they're linked (a WWI-era prequel centered on Pearl as a younger woman has already been shot), but the exaggerated artificiality of her appearance leaves one side of the young-old dichotomy in “X” sagging.

Still, this is strong genre filmmaking by West, a writer-director of horror films and thrillers. The photography by cinematographer Eliot Rockett is vivid — there's one arresting shot from overhead of an alligator trailing a naked Maxine during a swim. The actors are uniformly good. And by fusing two types of films that have long been bedfellows — slashers and pornography — “X” makes for a gripping shotgun marriage of genres.

“X,” an A24 release, is rated R by the Motion Picture Association of America for strong bloody violence and gore, strong sexual content, graphic nudity, drug use, and language. Running time: 105 minutes. Three stars out of four.

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Follow AP Film Writer Jake Coyle on Twitter at: http://twitter.com/jakecoyleAP