Gazing out over the wall that encircles Chicago at the end of “The Divergent Series: Allegiant,” Tris Prior longs for the time when she didn’t know what lay on the other side. Her imagination, like ours, had clearly been primed for exciting revelations beyond the realm of Veronica Roth’s juvie sci-fi franchise, which inexplicably switches allegories late in the game. What began as a massive, if astoundingly implausible sociology exercise — where citizens were sorted into character-specific factions, like Hogwarts first-years awaiting their house assignments — has morphed into a downright ridiculous anti-eugenics parable. And whereas Roth’s political subtext was previously rich enough to overlook the films’ second-rate action set pieces, her message has become so muddled, Summit has every reason to worry whether tween audiences (already down $20 million since the first movie) will remain allegiant to a weakening franchise through its forthcoming fourth episode, expected summer 2017.

Picking and choosing details from Roth’s complicated and somewhat controversial third novel, a trio of screenwriters new to the series deliver the first half of an artificially protracted two-part finale, which diverges not only from the source material, but also from where returning director Robert Schwentke left things in the previous film. There, as the music swelled, we saw “factionless” chieftess Evelyn (Naomi Watts) executing the dictatorial Janine (whose accomplices now face a similar fate) while the citizens swarmed en masse toward the wall, finally opened after more than 200 years.

Except, as “Allegiant” begins, the wall still stands and Evelyn’s heavily armed guards make every effort to keep the city cut off from the outside world, where, we’d been told, “Mankind waits for you with hope.” Running vertically up the cement surface, just five characters manage to cross the barrier here: Tris (Shailene Woodley, sporting yet another new hairstyle); her brother, Caleb (played by “The Fault in Our Stars” love interest Ansel Elgort); her actual love interest, Four (Theo James); Dauntless ally Christina (Zoe Kravitz); and the consistently unreliable Peter (Miles Teller).

The first movie actually began on the other side of this wall, panning across the evocative sight of a freighter abandoned in a field of tall green grass (the boat could still be seen in “Insurgent”). Now, in its place, Tris finds what looks like a Martian desert, where it rains toxic red water and the life expectancy is just two or three decades. A futuristic force field creates a second kind of wall, this one isolating the survivors of something called the “Purity War,” whose post-apocalyptic effects supposedly explain the radical change of setting. Picked up by armed soldiers, Tris and her team are brought to what remains of Chicago O’Hare Intl. Airport. (“What’s an airport?” asks Caleb, who might as well also ask, “What’s a nation?” These shut-in characters have an awful lot to learn.)

Finally, three films into the series, we get to discover why the city has been set up the way it is: At some indeterminate point in America’s future (and “Divergent’s” past), the government started fooling with human DNA, attempting to eliminate unwanted traits (such as a so-called “murder gene”) from its citizens. The effort backfired, and the new genetically modified population rose up in rebellion, reducing the United States to a radioactive wasteland and forcing the Bureau to take drastic (and totally non-scientific) measures.

In what sounds like an embarrassingly naive plan, the Bureau fenced in Chicago, filled it with genetically “damaged” citizens and imposed the faction system to ensure peace, effectively hoping that the human genome would heal itself. Meanwhile, using incredibly sophisticated surveillance technology, these outsiders tuned in to monitor every little detail, a la “The Truman Show.” If their goal was to find Divergents, it raises the question why they didn’t intervene during Janine’s genocidal reign — or now, as an unconvincing civil war brews between Evelyn and former Amity spokeswoman Johanna (Octavia Spencer).

It’s foolish to get too caught up in such questions, although asking audiences to turn off their brains basically reduces “The Divergent Series” to just another sci-fi action franchise — and not a very good one at that. With plot holes bigger than the chunks missing from its dystopian skyline (still the best visual effect in the wildly inconsistent, CG-dominated mix), “Allegiant” will seem awfully meager to those who so recently feasted on “Mad Max: Fury Road” … or “The Hunger Games,” or countless other superior examples of the genre.

What this more conventional installment does have going for it is a fresh “Tron: Legacy”-like score (from composer Joseph Trapanese) and a milder youth-skewing sensibility: The characters shoot what look like toy plastic guns, hitch rides in floating “plasma globes” and zoom about in bullfrog-shaped hovercrafts. Plus, the series never second-guesses the fact that its strongest characters are female, whether it’s role model Tris or rival leaders Evelyn and Johanna.

“Allegiant” introduces yet another potential villain, this one a superficially benevolent bureaucrat named David (Jeff Daniels, cementing the authority-figure typecasting seen since “The Newsroom”). David is obsessed with genetic purity, a concept he has incorporated into the decor of his immaculate white office, with its helix-shaped staircase and ivory-tower location, perched a hundred useless floors above ground. Apart from Tris, David doesn’t seem to care about any of the anonymous souls in Chicago — an attitude presumably shared by Schwentke, who gets some of the worst performances from extras in any film of this scale.

Although “Allegiant” does recapture the original film’s sense of constantly discovering and adapting to fresh information, audiences no longer identify with anyone in particular. For those who thrilled at the idea that she was somehow special, Tris has been pushed aside, while Four goes out on dangerous missions to “the Fringe,” visiting tent cities where David abducts children for purposes the movie never satisfyingly explains. When David’s plan goes askew, he relies on Peter to release memory gas into Chicago, resulting in laughable scenes in which the awkward aforementioned extras try to outrun clouds of orange smoke.

While it’s up to Tris to stop David from wiping everyone’s memories, that plot is virtually the opposite of the more complicated sacrifice Roth imagined for her in the book. It’s as if the filmmakers have lost interest in Tris — and who can blame them? Here we realize that by opening the box at the end of the previous film, Tris revealed herself to be not “divergent,” nor special, but effectively the same genetic state as everyone sitting in the movie theater — which is to say, purely average. Maybe the fourth film will recover what made Tris such a unique heroine, although in contrast with the divided last chapters of “Harry Potter” and “The Hunger Games,” there’s no epic villain for us spend the intervening year rooting against. And with no real cliffhanger to keep us interested, after “Divergent,” “Insurgent” and “Allegiant,” we can’t help feeling a little Impatient.