There’s something almost great about the Purge movies. Unlike so many other recent horror franchises, the creatives involved flirt with larger socio-political issues as well as providing the murderous mayhem that genre fans crave. The nifty, troubling concept, that sees all crime legalized for one 12-hour period a year, has allowed for a number of entry points for conversations about class, race and capitalism. Yet still, not one of the three films thus far has managed to take this combination of grisly nihilism and bleak social commentary and turn it into something that truly soars.

After 2016’s The Purge: Election Year, the most overtly political entry yet, the story is going back to the beginning to trace the decision-making process that led to the inception of the tradition. With America in a believably fraught state (the introductory montage informs us of rising unemployment, mass opioid addiction and an escalating mortgage crisis), the presiding party, the nationalist, ultra-religious New Founding Fathers of America, develops an idea that could potentially reduce skyrocketing crime rates: what if everything was legal for half a day, once a year? Would the annual release of anger and violence lead to criminal abstinence for the remaining 364 and a half days?

To test out their thesis, a pilot scheme is arranged for those living on Staten Island. If citizens don’t want to get involved they can leave for the night but for those willing to stay, there’s a financial benefit and, inevitably, the experiment attracts those within poorer communities. But while psychologist Dr May Updale (Marisa Tomei) predicts that it will be a cathartic release of hate and violence, in actuality, residents mostly choose to party instead. “Human nature does not obey the laws of politics,” she says. For those in charge, this isn’t good enough and to achieve the desired result, they send in trained killers to stir up trouble. Local crime boss Dmitri (Insecure’s Y’Lan Noel) and his childhood friend, activist Nya (Superfly’s Lex Scott Davis) are then forced to defend their neighborhood and survive the night.

As patchy as the series may have been, there’s still something admirably dogged in writer-director James DeMonaco’s attempts to expand not only the Purge universe but the mythology embedded within it. Such fan service is reminiscent of what was attempted in the far more convoluted Saw franchise and while there’s repetition, there’s still inventiveness. With each film, DeMonaco has become more aware of how the structure can be used as a vessel for some real world commentary and with The First Purge, he’s written his most politically tinged film to date. He’s also retreated from the director’s chair for this installment, recruiting Gerard McMurray, whose frightening debut Burning Sands took a stark look at hazing within a black fraternity.

Each chapter has also grown in anger and The First Purge bristles with fury. From the second film onwards, the franchise has posited black characters in key roles and here, they are the majority, making up the largely impoverished test subjects. The setup, using footage from real life riots, builds a picture of a fractured, racially divided country in need of a dramatic solution (“The American dream is dead” spouts the NFFA). There’s even a cameo from CNN’s Van Jones playing himself as he wrestles with the morality of the experiment, interviewing its architects and questioning their decision to pay those willing to take part. It’s a secret form of depopulation, a way of eradicating those who the government no longer wants to pay for, an undeniably arresting idea for a wide-releasing horror film.

It sits alongside other smart kernels of ideas. The film flips between the streets and the white officials overseeing from their offsite tower and early on there’s the suggestion of a discrepancy from what they assume will happen and what actually does. The black community doesn’t resort to violence as they expected but instead they party and protect one another. But for every attempt made by DeMonaco to provide social critique, there’s a clumsy decision working in direct opposition. The script is riddled with hopelessly on-the-nose dialogue, most of which, unfortunately comes from Tomei, stranded with a thankless, dim-witted role as the Purge-creator who is then appalled at the violence it leads to (there were huge guffaws in the audience when, late stage, she asks “What have I done?”). There’s a shallow cheapness (from the invented TV networks CNB and WOQA to the hammily performed comic book politicians) that often makes the riskier, topical choices feel exploitative.

It’s a film clearly made with one eye on the news and it’s stuffed with increasingly queasy nods to what’s been afflicting America. White men dressed in the same getup as Charlottesville protesters shoot up a church full of black families. Klan members attack black-owned businesses. White police officers take it in turns to beat an unarmed black man with batons. A gestapo-esque leader guides a group of soldiers around a tower block, killing everyone they find. It’s unpleasant and deliberately so, a visceral assault that drives home a message of indignation at how black lives are treated. There are also more direct nods to Trump with a jab about Russian meddling and one character attacking a potential rapist calling him a “pussy-grabbing motherfucker” (the film’s marketing has also been Trump-baiting in the extreme). But all this anger is delivered in a questionable package, surrounded by conflicted messaging.

The faux-earnest soapy conflicts and cartoonish view of both political machinations and the criminal underbelly adds cheese to the aforementioned grit. While we’re told the villainous NFFA is funded by and endorsed by the NRA, we’re also bombarded with fetishistic imagery of guns, lovingly, lingeringly filmed. The finale in particular is so bullet-strewn that it plays out like a facsimile of an 80s action film, even down to a musclebound Noel in a vest, possessing Arnie-esque commando skills. Guns are shown to be an invaluable tool in the line of defense against invaders and the final messaging appears to be: if civil war is on the way then you’d better make sure you’re armed.

Clumsy attempts at comedy are weaved in to try and alleviate the remarkable grimness but all it really does it add to an uneven tone. By the time black characters are given the agency to fight back, there were mass audience cheers greeting the catharsis but the victories are minimal, arriving after so much dour brutality. The First Purge isn’t going to be the last Purge (a 10-part TV series arrives in September) but the premise remains awkwardly handled, a surer hand needed to balance the issues at play. It’s hard not to admire an attempt to bring tough societal trauma to the multiplex but in this particular package, it’s even harder to recommend.

• The First Purge is released in the US and UK on 4 July