2014's "The Maze Runner" (based on the first in James Dashner's three-part YA fantasy book series) separated itself from the onslaught of dystopian book-to-movie adaptations such as "The Hunger Games" and "Divergent" by opting for mystery over exposition, told from the perspectives of characters stranded in a labyrinth-surrounded glade who had no memories beyond their first names. Their discoveries were the audience's, happening in intriguing, layer-peeling tandem. For second installment "The Scorch Trials," returning director Wes Ball and screenwriter T.S. Nowlin pick up exactly where the last left off, the films seamlessly seguing into each other. Unfortunately, the story this time pales in comparison, feeling shopworn and less vital as it tramples across sands and tropes. Much of the film consists of introductions to new characters, tidy descriptions of who they are, and explanations of what is going on—elements that were quickly handled up front in "The Maze Runner" or more organically disclosed as the film progressed. Also sorely missed: Blake Cooper, whose youngest-of-the-group Chuck was the arguable heart and soul of the predecessor.

Human rats who have adeptly fought off the deadly Grievers and found their way out of the maze in which they were trapped, Thomas (Dylan O'Brien), Teresa (Kaya Scodelario), Minho (Ki Hong Lee), Newt (Thomas Brodie-Sangster), Frypan (Dexter Darden) and Winston (Alexander Flores) are promptly helicoptered to a maximum security holding facility where they discover there are other teenagers just like them. Each day, a short list of names are revealed, the lucky few supposedly transported to a safe zone far away from the apocalyptic landscape outside. Thomas isn't so sure this is the case. Frustrated that Teresa has been cryptically separated from the group, he is already mistrustful of Dr. Janson (Aidan Gillen) and the rest of the medical and military personnel when another kid, Aris (Jacob Lofland), reveals to him their diabolical operation: they are harvesting maze survivors' bodies in hopes of finding a cure for the catastrophic flare virus. With no time to spare, Thomas finds Teresa and escapes with her and the rest of their friends into the harrowing desert scorch, an unforgiving wasteland where Dr. Janson warns they will not last one day.

"The Scorch Trials" is arresting it its first and third acts, drawing one back into the evolving, handsomely plotted fold and capping the proceedings off with a major curveball and the much-anticipated reappearance of Patricia Clarkson (2011's "One Day") as morally shady WCKD head Ava Paige. The sluggish, overly familiar middle hour-plus, however, sputters on fumes. Aesthetically, the scorch is a dull place of ramshackle buildings and copious dust and sand. The initial sight of the infected, zombie-like cranks, seen briefly from behind as they barrel across the nighttime dunes, overwhelms with ominous suggestion. Once they enter into the story proper, disappointment sets in when they turn out to be rotting, cannibalistic, standard-issue copies of the baddies from 2003's "28 Days Later," 2004's "Dawn of the Dead" remake, and TV's "The Walking Dead" rolled into one. Sporadically diverting chase scenes ensue, as does an action sequence involving teenage resistance fighter Brenda (Rosa Salazar) and a slowly cracking pane of glass so shamelessly plagiaristic of a nearly identical scene in 1997's "The Lost World: Jurassic Park" it has to be seen to be believed.

In terms of look and style and the core cast members, "The Scorch Trials" is at one with the superior "The Maze Runner." As central hero Thomas, Dylan O'Brien (2013's "The Internship") continues to appealingly carry the series. Visual effects are solid, rarely looking too computer-generated for their own good. Director Wes Ball has proficiently imagined a world stricken by disaster, one that warrants further examination. The main trouble has occurred at the scripting stage, the film too frequently falling into the trap of explaining rather than showing. With the plot failing to substantially advance and the pacing curiously inert even when the characters are hustling, this middle chapter cannot quite escape the feeling that it is pre-finale filler. Without Chuck, the film also cannot come close to matching the raw emotions of its precursor's gut-punch of a climax. "I never thought I'd say this, but I miss the glade," Frypan comments midway through "The Scorch Trials." He is more right than he knows.