It’s prequel time for the “Purge” franchise: this fourth installment explains how the once-a-year night of legally permissible murder began. While even more heavy-handedly didactic than the last entry, “Election Year,” it’s somewhat less nihilistic, opining that the inclination to brutal aggression was not inherent in the population, but had to be incited by a malevolent government that had devised the gruesome business for its own nefarious purposes.

“The First Purge,” directed by series newcomer Gerard McMurray, is also efficiently made by comparison to the rather messy predecessors helmed by creator-writer James DeMonaco. In the end, though, it’s still the same combination of mindless violence and crude social commentary, appealing to the worst instincts of the audience in much the same way that the fictional purge appeals to those of its would-be participants.

It’s some unnamed year, and the newly-elected leadership of the U.S.A., headed by President Bracken (Ian Blackman), has an idea about how to make America great again by establishing the purge, conceived by behavioral scientist Dr. Updale (Marisa Tomei) as a sociological experiment but intended by her elite bosses a means of balancing the budget and reducing crime by weeding out undesirables (i.e., the poor, minorities and other troublemakers). The administration decides to stage an experimental test-run on Staten Island, offering cash payments to those who sign up to remain and further financial incentives to those who actually take part in the violence.

Some, like psycho drug addict Skeletor (Rotimi Paul), gleefully look forward to the event. Others, like neighborhood nag Nya (Lex Scott Davis), oppose it, while her naive young brother Isaiah (Joivan Wade), sees it as a means of getting back at people like Skeletor. Then there’s Dmitri (Y’Lan Noel), the local drug kingpin, who just doesn’t want his operation interfered with.

When the night begins, the level of carnage, observed by operations director (Patch Darragh) from a command post, is depressingly low. That leads him to resort to a backup plan, which involves inserting vicious mercenaries, dressed as cops, KKK members and neo-Nazis, to stir things up. Meanwhile Dmitri, who had planned to stay out of the fray, turns heroic as he sees his position, and his turf, threatened, and he turns Rambo-like hero, helping out older community leader Freddy (Steve Harris) against a horde of hooded Klansmen and then rushing to save his former squeeze Nya and those who have joined her in the high-rise where she lives with Isaiah against the SWAT-like attackers clearing out the building with extreme prejudice (of every sort, since the soldiers are pretty much all-white and their victims not).

The level of blood-spurting and gore gets pretty high as the mayhem grows, but there are occasions in which McMurray pulls back a bit. Nya, for instance, first repairs for the night to a local church, which the pastor has opened up for those seeking sanctuary from the expected marauders, thus setting up what’s essentially a human smorgasbord for consumption. But although the hastily assembled congregation does, in fact, get wiped out (or nearly so), we’re not actually shown the slaughter. And while there are individual killings on the streets, the picture often opts for hair-breadth escapes and chases instead, with the occasional “gotcha” moment to spice things up. When it comes down to the last-act assaults, moreover, they’re of a sort that DeMonaco surely learned to construct from his work on the remake of “Assault on Precinct 13” years ago.

As is usual in such fare, the acting is strictly functional, though Noel makes a stalwart hero and Mugga adds some levity as a tart-tongued neighbor of Nya’s. Paul sneers and threatens with abandon, but Tomei tries to hide beneath a horrendous blonde wig as the initiator of the experiment; it’s certainly a low point in her career. Tech credits are okay, with cinematographer Anastos N. Michos employing haze and shadow to some effect and editor Jim Page keeping the action clear. Kevin Lee’s score is utterly generic.

It’s surely time that the “Purge” franchise was itself purged from our cinematic bloodstream, but as long as these modestly-budgeted bloodfests keep bringing in the bucks, that won’t happen.