According to mow reviewers (The Wrap, Variety), "The Divergent Series: Allegiant" just brought in the same cast of characters to do what they have they done in the very first installment of the movie franchise. The setting might have changed, but the characters all have the similar characteristics and feelings for each other. Tris (Shailene Woodley) remains the earnest and athletic girl that she was since "Divergent." Peter (Miles Teller) still betrays the group every chance he gets. This time around, the movie is set in a "more sci-fi gloss than post apocalyptic grunge," as the Wrap reported.
The problem is that the producers tried to deviate so much from the book that the point of the book vanished, as reported by Variety. "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," the Variety reported. However, doing so made the whole thing all the more confusing.
The second movie ended with "factionless" chieftess Evelyn (Naomi Watts) executing the dictatorial Janine while the citizens ran toward the wall, finally opened after more than two centuries. However, the third movie opened with the wall still standing and Evelyn's heavily armed guards are doing all that they can to keep the city cut off from the outside world. Expectedly, only the main characters were able to get out of this wall: Tris, her brother, Caleb, Four (Theo James), Dauntless ally Christina (Zoe Kravitz); and the sneaky Peter.
The outside world looks like nothing the way it was introduced in the first movie either. The first movie began on the other side of the wall, where viewers are treated to evocative sight of a freighter abandoned in a field of tall green grass. The boat was still seen in "Insurgent" so there was a continuity with that at least. However, on "Allegiant," the outside world looks like a Martian desert, raining toxic red water and people only living up to two or three decades.
There is also futuristic force field that acts as a second kind of wall, separating the survivors of the so-called "Purity War." The war is said to be the main reason for the post-apocalyptic effects or change in setting for this third movie. Still, many would certainly wonder if they're watching the movie they intended to watch.
There are many more deviations, but the main contention was that the movie is very different from the book. The characters might still be the same, but the main substance vanished. "The is virtually the opposite of the more complicated sacrifice Roth imagined for her in the book," Variety said.
Here is a trailer of "Allegiant:"