Third in the Divergent series (and final), this one didn't fall flat like some of the other 3rd books in trilogies (looking at you, Mockingjay).
Tris, Tobias and crew are stuck inside Chicago where Evelyn, Tobias' mother, leads the new city, by forbidding factions and keeping the factionless armed and patrolling the city. Just another tyrant on the wall.
The Allegiant is formed, a group of people who want the old way of life and who want to follow Edith Prior's video of leaving the city walls. A small group of Divergent, including Tris, Christina and Tobias, head out and discover..... an airport. Otherwise known as the government who is using people as experiments. Now we're getting creepier.
Allegiant explains the purpose of the Chicago experiment and genetically "damaged" vs genetically "pure". It all got very Hitler/Nazi-esque to me, which I suppose is what it was supposed to do.
Someone has to rise against, and they do, with some dire consequences to some main characters. I'll give props to Roth on the ending. Quite powerful.
Since I refuse to spoil, I'll finish with: this is a great ending to a great series. Bravo!