The fourth Terminator picture in a fast-paced but slow-moving series (the first installment came in 1984, the second in 1991 and the last in 2003) is an attempted rebooting that, rather like the new “Star Trak,” takes us back to the beginning—in this case the war between men and machines that lurched into the past with the time-travelling action of James Cameron’s first two pictures and Jonathan Mostow’s third. So it’s a prequel that actually occurs in the future (2018, to be precise), when John Connor—the human hero whose mother the original Terminator tried to kill in the first movie (and whose father arrived to prevent it), and whom other Terminators targeted himself in the other two films (and in the recently-cancelled television series)—is the grown-up resistance leader that the machines will later (that is, earlier) seek to eliminate even before he’s born.

If all that sounds confusing, you’re obviously not familiar with the whole Terminator mythology, but not to worry—“Salvation” refers to it in a way that will satisfy committed fans, but doesn’t require a master’s degree in Terminatorology. McG’s movie is basically just a full-bore actioner replete with special effects, man-on-machine battles, chases and explosions—endless explosions, it often seems—with, of course, a message about what it means to be human. That thread centers on the script’s major new character, Marcus Wright (Sam Worthington), a hard-boiled killer executed in 2003 who gets a second chance to do something heroic when he’s reborn as a half-human, half-machine cyborg Terminator prototype who must decide which of the halves he’ll follow in the effort to save not only Connor’s “future” father Kyle Reese (Anton Yelchin) but Connor himself. So mankind’s fate rests in the hands of this hybrid, and Connor’s ability to see the humanity in him.

That conflict is really the heart of “Salvation”—literally as well as figuratively, it turns out—and it means that while Christian Bale is top-lined in the cast, it’s really Worthington who’s the centerpiece of the picture. The two men actually look very much alike, down to their photogenic stubble, which plays up the implied theme of strained brotherhood that underlies the tale. (The fact that the young Reese is also a sort of father to Marcus, introducing the bewildered fellow to the resistance, reinforces the idea.) But the unhappy fact is that neither the Connor-Marcus-Kyle story thread nor the romantic overlays (Connor is loved by his wife Kate, a medic—I think—played by Bryce Dallas Howard, and Marcus falls in with resistance pilot Blair Williams played by Moon Bloodgood) manage to give much of an emotional dimension to the picture; they’re simply overwhelmed by the action set-pieces.

And quite frankly on that level the movie disappoints. It’s expertly made, of course, as one would expect of any such product of the Hollywood blockbuster factory. But it’s hobbled by a number of factors. One is that the vision of a dystopian future in which it’s set is by now pretty old-hat. It looks much like the devastated locale we’ve seen the “Mad Max” series and countless imitators. Even the Terminator machinery has a hand-me-down quality, reminiscent not only of the earlier installments of this franchise but of the stuff one finds in more colorful form in “Transformers.”

The picture also lacks the saving grace of humor; it takes itself deadly seriously, a fact that takes a toll on the performances, especially Bale’s. He’s all gruff, simmering intensity. Using his “Batman” growl, he breaks his impassivity only with bursts of righteous anger, and frankly makes Connor a pretty standard-issue action figure. Even when he’s given the opportunity to generate a laugh—by repeating a famous line from the first movie, or confronting a fearsome Terminator with a very familiar face, he doesn’t generate much more than a mild chuckle. Newcomer Worthington is more interesting, as his character is conflicted and more passionate, but he’s still a mite stiff. Only Yelchin seems really loose and human. The females are a pretty anonymous bunch. Bloodgood’s fighter pilot is apparently intended as a tomboyish Sarah Connor substitute, but isn’t much more than adequate, Howard barely registers, and Helena Bonham Connor is surprisingly anemic as the scientist who first meddles with a human-machine hybrid (of course, on her behalf it must be said that her character’s very ill). Even Jane Alexander, in a virtual cameo as a kindly human survivor, makes little impression. Then there’s Michael Ironside as the technical head of the resistance. He’s his usual stentorian self. (It’s unkind, but not inaccurate, to note that his presence often drags down the movies he’s in.)

“Salvation” is, of course, technically advanced, although quite frankly the Terminator effects lack much sense of surprise or elegance. And while McG shows that he can choreograph fight scenes confidently, his work overall isn’t particularly notable except for its energy. Even Danny Elfman’s score lacks the idiosyncratic panache he usually brings to his music; it sounds as though it might have been written by anybody.

So “Terminator: Salvation” does pretty much what you’d expect but little more, and its sullen efficiency doesn’t make up for its lack of imagination. You leave the new “Star Trek” feeling exhilarated at the revival of an old franchise. This movie you depart thinking, “Been there, seen that.”