“The First Purge” follows the franchise tradition of taking what could have been an intriguing idea, staying with it for about the amount of time it takes to eat a small popcorn, then slicing and dicing it and gunning it down in an orgy of over-the-top violence.

So they’re consistent, if nothing else. Anything interesting gets drowned in a bloodbath.

James DeMonaco, who wrote and directed the first three films in the series, cedes the director’s chair to Gerard McMurry; DeMonaco wrote the script. It’s a prequel, explaining the premise of the original film and those that followed — namely, that for one night a year, all laws are suspended. Everything is legal, up to and including murder.

The country has changed a lot in the past couple of years. Things that once seemed unthinkable now seem plausible, yet the idea of a Purge thankfully still seems a little far-fetched.

The last film in the franchise, “The Purge: Election Day,” came out in the summer of 2016, and played with the concepts of the political moment before just killing everybody. It featured a woman running for president of the United States on an anti-Purge platform. She recognized that, while unemployment and crime rates plummeted, the Purge in reality was a way of culling the population of poor people and minorities.

“The Final Purge” purports to explain how it all came to be. In reality it dispenses with the causes — plunging stock market, another housing crisis, spiraling unemployment, civil unrest — in a quick montage. All this leads to the rise of a third political party, the New Founding Fathers of America, or NFFA, basically a bunch of white guys who want to return the country to whatever supposed glory it enjoyed when people who looked like them ran things.

“The American dream is dead,” the new president says. “We will do whatever it takes to let you dream again.”

These wingnuts get the great idea to conduct an experiment: the no-laws-for-12-hours idea. It won’t be nationwide. Instead it’ll take place only on Staten Island. You don’t have to stay on the island during the experiment, but there’s $5,000 in it for everyone who does. Critics protest that this unfairly targets minorities and the poor, but the NFFA supposedly has science on its side, courtesy of Dr. Updale (Marisa Tomei, like the poor Staten Island residents, looking for a payday, nothing more). She’s the architect of the big experiment, and argues that it’s a freeing experience.

Meanwhile Isaiah (Joivan Wade) is starting to sell drugs on the corner, and runs afoul of full-on psychopath Skeletor (Rotimi Paul). Isaiah’s new job enrages his sister, Nya (Lex Scott Davis), who used to date Dmitri (Y’lan Noel), the drug kingpin in the neighborhood. He may sell drugs, but Dmitri is the thoughtful voice of reason in what’s becoming a mad world. He orders his troops to sit the Purge out.

All this is mere prelude to the big night itself. The media dutifully sensationalizes it, the politicians embrace it and the people involved make their decisions. To help keep track of what’s going on, the government offers participants weird-looking contact lenses, which help officials see what’s going on and keep track. Like the movie itself, it’s briefly interesting — the lenses make everyone who’s purging look scary in the dark — but it’s eventually abandoned.

Alarms sound and the Purge is on. And then … nothing. Or at least not much. Not enough violence to satisfy Arlo Sabian (Patch Darragh), the smug chief of staff, certainly. Dr. Updale finds the lack of mayhem intriguing — some people are even having Purge parties, dancing in the street. But to Sabian and his superiors, it’s not enough.

Despite a live broadcast of Skeletor’s first kill — good ol’ Skeletor, you can count on him — there doesn’t seem to be a lot of interest in people actually killing other people for no good reason.

Until, suddenly, Sabian makes a few phone calls and the violence spikes. Hmm. What could possibly be going on here?

It doesn’t take a genius to figure it out. And it doesn’t really matter, anyway. From here the movie is all about Dmitri, Nya, Isaiah and their friends trying to survive, anyway.

As with most prequels, there’s ultimately not a lot of suspense, since we know what’s going to happen in the next installments. Tell us something something to care about.

★1⁄2

Universal Pictures presents a film directed by Gerard McMurray and written by James DeMonaco. Rated R (for strong disturbing violence throughout, pervasive language, some sexuality and drug use). Running time: 99 minutes. Opens Wednesday at local theaters.