Ever since it began with James Cameron’s brisk, brutal B-movie in 1984, the “Terminator” franchise has followed the title figure’s most famous line from that picture—“I’ll be back.” Sequel after sequel followed, getting bigger and blowsier all the time—as well as more serious and stentorian. By the time of the last one, subtitled “Genisys,” the time-changing convolutions and general air of pretension had rendered the series well-nigh unwatchable.

Enter Cameron himself to rejuvenate the property. Taking over the production reins, he’s the force behind “Dark Fate,” a reboot that in effect sweeps aside the last three installments about John Connor and effectively presents itself as direct descendant of 1991’s “Terminator 2: Judgment Day,” which, of course, he directed. He doesn’t take on that duty this time around, however, handing the helming chores off to Tim Miller, of “Deadpool” fame.

Any idea that Miller might bring a sassy, tongue-in-cheek tone to the franchise is quickly disappointed, though. “Dark Fate” escapes the utterly gloomy grimness of “Genisys” and its predecessor “Salvation,” but not by much; apart from some deadpan levity provided by Arnold Schwarzenegger as a much mellowed older version of the initial Terminator T-800 model, it’s a pretty humorless piece of action-movie mush.

It’s also as much a recycling as a reboot, bringing along Linda Hamilton as well as Arnold to reprise her role as Sarah Connor, now as hardbitten a old broad as Jamie Lee Curtis was as the aging Laurie Strode in David Gordon Green’s “Halloween” last year.

But the imperiled woman this time around is Dani Ramos (Natalia Reyes), an auto assembly line worker in Mexico City. She’s barely introduced before a new and improved Terminator, the Rev-9 model (Gabriel Luna), comes after her. But a protector has been sent in the person of Grace (Mackenzie Davis), an “augmented” human with some super-powers but also susceptible to injury and exhaustion. She’s not really a match for the Rev-9, so it’s a good thing that crusty old Sarah turns up to help out.

The trio now must cross the U.S. border to reach Laredo, the source of a signal that has been sending Connor information about the arrival of new Terminators so that she can blast them away. After a melee at a detention center, they reach a remote ranch outside the Texas town, where they find Carl (Schwarzenegger), a draper who’s a humanized version of an old T-800. Naturally Sarah bears a grudge against him (it?), but reluctantly accepts him as a partner in their struggle to save Dani from the Rev-9.

The whole of “Dark Fate” is an extended chase punctuated by protracted fight sequences. The series begins with a highway pursuit in Mexico City and continues through the detention center sequence before winding up at a dam, where self-sacrifice becomes the norm in defeating the unstoppable foe. And although that initial car chase is pretty ridiculous—with long leaps in which Grace and Rev-9 always start out as the actors but abruptly turn into cartoonish CGI figures in mid-air (a trait common to all such CGI-assisted “stunts”)—things get more and more absurd as the story proceeds. Our heroes manage to acquire a helicopter, and later a giant military cargo plane, with a minimum of fuss, along with some special electronic bomb that’s supposed to annihilate the robot. (The script brushes aside any queries about this by having Sarah say, “I know a guy”—who turns out to be an army officer.)

Matters are made more obscure by the fact that the script never really makes clear what sort of reality we’re dealing with here. It appears that Sarah and her son John had effectively derailed the Skynet future of the first two “Terminator” movies (and the next three installments too), and the foe this time around is an equally evil AI empire called Legion that emerged in an alternative future glimpsed in flashbacks showing Grace’s past and her transformation into some sort of meta-human. Or maybe not, because Rev-9 appears to be just an updated T-1000—so perhaps Legion is a successor-robo-state to Skynet.

Whatever the case, as potential victim and prospective savior Dani Reyes doesn’t make much of an impression (one scene, in which she supposedly exhibits her leadership skills, falls flat), and while Davis exhibits an fine physique and considerable dexterity, she fails to make Grace a compelling character (though admittedly the script doesn’t give her a lot to work with). Luna brings an appropriately hollow persona to Rev-9, though he doesn’t begin to match the grim menace with which Arnold endowed the original Terminator (or Robert Patrick the T-1000 with in “Judgment Day”).

As noted, the effects here—a major element in this sort of would-be blockbuster—are okay but not outstanding; as in so many cases, the action sequences tend to grow rather murky and unfocussed the more prolonged they become (and they are very long here). Otherwise the technical credits—Sonja Klaus’s production design, Ken Seng’s cinematography, Julian Clarke’s editing—are fine. But if the detention-center sequence, along with the preceding scene of the heroes’ apprehension, was designed to add a touch of social commentary to the mix, it fails, coming across as more tasteless than compelling, especially given that it abounds with stereotypes (drawling Texas lawmen, for starters).

There’s some fun in seeing Hamilton and Schwarzenegger face off again, but otherwise this “Terminator” is pretty much a dud. Despite an ending that seems to augur future installments, don’t be surprised if “Dark Fate” proves as definitive an end to this long-running saga as “Dark Phoenix” was for the X-Men series.