Sometimes running times can tell the story. The first episode in the “Maze Runner” trilogy, based on the YA novels by James Dashner, was 113 minutes, and while hardly a classic, it moved swiftly enough through the inanity to make it palatable. The second movie, “The Scorch Trials,” was longer at 131 minutes, but contained enough incident to support the increase. Now comes the final installment, “The Death Cure,” which, at 142 minutes, drags on repetitiously, its simple narrative unable to sustain the unconscionable length. Despite lots of bombastic action scenes, individual face-offs and self-sacrificial turns on the way to an idyllic conclusion, it lumbers rather than sprints to the finish line.

In essence the entire picture is about the efforts of the three remaining “Gladers” (survivors of the Big Maze from the first film) to rescue one of their own, Minho (Ki Hong Lee) from the clutches of WCKD, the sinister group using these immune youngsters to try to devise a cure for “The Flame,” an epidemic that is turning most of humanity into zombie-like flesh-eaters. Thomas (Dylan O’Brien), Newt (Thomas Brodie-Sangster) and Frypan (Dexter Darden) are the trio, but they are aided by some outsiders, most notably Vince (Barry Pepper), Jorge (Giancarlo Esposito) and Brenda (Rosa Salazar), in their mission, along with a surprise ally, a major character from the first picture who is effectively resurrected here but whose identity will not be revealed to avoid spoiling the surprise (though readers of the books will certainly know who it is—and a glance at the cast list provides an obvious clue).

After saving a train car full of young immunes from WCKD in dashed hopes of finding Minho among them, the trio set their sights on the walled city in which the evil operation has its laboratory and their friend is serving as a guinea pig. But to work their way into the lab, they will need the help of their erstwhile comrade Teresa (Kaya Scodelario), who has defected to WCKD, though of course she is conflicted about being in the employ of its boss Ava (Patricia Clarkson) and her nasty enforcer Janson (Aidan Gillen).

Much of “The Death Cure” is devoted to acts of derring-do and clashes with Janson and his army of storm troopers. Unfortunately, while the action is decently staged by director Wes Ball and his cohorts, and shot fairly nicely by cinematographer Gyula Pados, it becomes tiresomely repetitive. The train sequence, for example, involves a large grappling hook lowered from a futuristic plane, and it’s pretty exciting (even if the actions of the train’s protectors are typically inept). Ball apparently liked the idea so much that he repeats the same stunt toward the close, except that the vehicle being lifted is different. It’s not nearly as effective the second time around.

Then there are the repeated face-offs with Janson. People are constantly holding guns on one another, but rather than firing, they speechify until something happens to allow our heroes to get away for the next encounter. The number of close calls and hair’s-breadth escapes in the movie is astronomical, and frankly boring.

And there are plot points that seem insufficiently explained. One of the immunes suddenly begins to turn into a crank, as the infected undead are called, but there’s no clear reason why that should happen (and at the most inappropriate moment, though it does lead to an exhibit to self-sacrifice that’s one of the better elements of a very long climax). At the close Janson realizes that one person is the key to curing those afflicted with The Flame, but then tries to kill the person. (He’s already murdered somebody else for no discernible reason.)

The result of all this is that “The Death Cure” gets sillier and sillier as it draws to a conclusion all too slowly despite multiple riots, explosions, fights, bullet wounds and chases. Piling climax upon climax makes not for excitement but irritation.

That said, the movie is generally well-made from a technical standpoint, with the effects and stunt crews doing good work while editors Paul Harb and Dan Zimmerman do a good job livening up the action scenes even as they proliferate mindlessly in the last act.

The acting for the most part is serviceable, with O’Brien—who suffered a severe injury from an accident on the picture’s set, necessitating a substantial production delay—making a likable, if somewhat ineffectual hero and Scodelario okay as his emotional love interest. But Gillen is a smirking bore as Janson, Clarkson so underplays Ava that she practically disappears, Pepper is just conventionally gung-ho, and Walton Goggins, caked in makeup, is almost unrecognizable as the voluble leader of the city’s opposition to WCKD,. On the other hand Esposito has fun as Han Solo-esque Jorge, and Salazar is physically impressive as his protégé. Most of the others are okay but unremarkable.

The major exception is Brodie-Sangster, who gives what is certainly the best performance as the loyal Newt. He offers a degree of intensity that periodically rouses the picture to life when it threatens to sink into tedium.

This may be the last gasp in the genre of YA dystopian future movies—most of its competitors, save “The Hunger Games,” having collapsed after a single episode or expiring as a result of the long haul, like “Divergent.” It’s a pity that this final installment doesn’t measure up to the promise of the earlier ones, but we can at least be thankful that the makers of the “Maze Runner” franchise didn’t follow the example of many similar projects and divide the final part of the trilogy into two pictures. Given that the present movie suffers from a lack of complexity, that would have been a true travesty.

It’s also to be noted that despite its length, “The Death Cure” doesn’t bother to provide any background on what transpired in the previous two installments. Newcomers to the series will need to do a lot of guesswork to divine a lot of the plot points; on the other hand, there probably won’t be many of them.