“The Purge” is a paranoiac’s nightmare come true. It turns the main attraction of a gated community — safety in locked-down homogeneity — on its bloodied ear.

It’s 2022 in America, and crime is at an all-time low. The country’s “new founding fathers” think this is because they have decreed that one night a year, from 7 p.m. to 7 a.m., nothing is illegal and cops have the night off. Anyone can murder, rape and assault with abandon, facing no consequences, to “reduce the aggression we all have inside us.” The film opens with footage of past years’ atrocities in the name of citizens getting their ya-yas out, so they’ll float through the rest of the year on a cloud of calm goodwill.

Sounds like the perfect environment to make money as a security-system expert, which is what the prosperous husband and father played by Ethan Hawke does. But not long after he returns to his requisite spacious home, gorgeous wife (Lena Headey of “Game of Thrones”) and two kids, a compassionate act blows a huge chunk out of the armor.

His tween son temporarily disarms the mansion’s defenses to let in a black man — who, in this setting, might as well have the words “noble savage” tattooed on his forehead — being pursued by a teen gang sporting prep-school jackets and grotesquely grinning masks. The group’s leader, a blond guy doing his best impression of Heath Ledger as the Joker, explains through the intercom that unless the human prey is tossed back outside, the whole family will die.

It’s about then that what is promisingly foreshadowed as a high-drama exploration of a worthy psychological theme — fear of the Other — devolves into the assumption that indiscriminate bloodlust lurks just below the epidermis in all of us, waiting for the pinprick that transforms us into the Manson Family.

Writer/director James DeMonaco tries to obliterate the credulity gaps with frenzied pacing, and somewhat succeeds, judging by several moments when the theater went dead silent, without even a candy-wrapper rustle.

But the preppy thugs’ masks (which bear an unfortunate resemblance to the precisely carved face of Abercrombie & Fitch CEO Michael Jeffries) soon become more annoying than frightful. It’s legal to kill, so why bother to hide identities? For such a smart kid, why does the son keep waving his lit flashlight around when he knows he’s being stalked? Why doesn’t a security expert have an impenetrable safe room? And a final, easily predictable twist involving some Stepford-style neighbors falls flat.

While you wouldn’t want the 85-minute killfest to last longer, its premise requires more sophisticated layering for the audience to really get on board with the notion that people leading very comfortable lives will suddenly go on a murder bender just because they can.

The only real takeaway here is a bit of implied advice: If you happen to notice your all-American neighbor honing a giant glistening machete in his back yard, it’s time to think about moving.