This series did not go well for me. The first book was interesting, but the second and third books sucked.
Tris, Four and some of their friends (Divergent and normals alike) leave the city when they find out there are other people in the world after the factionless takeover. They end up at O'Hare with a government bureau, where they're told the whole city of Chicago is a giant experiment, designed to heal genetic damage and the whole factions thing is a supposed to somehow to heal the damage. The Divergent are supposedly genetically "pure" (GPs) because their genes have healed, while everyone else is "damaged" (GDs). GP's are obviously better, so the GDs live in squalor and serve the GPs. That's it. That's the big reveal. No explanation of why Tris has magical serum resistance, it just must be that her genes are pure.
The first problem is that this book is split between Tris' point of view and Four's, so some chapters are from her POV, others are his. The problem is that they switch back and forth inconsistently, so it's REALLY hard to tell who's talking. Tris and Four then resume their idiocy. The swear that they'll never, EVAR lie to each other again, then Four learns he's genetically damaged and goes into pouty, 'poor pitiful me' mode. So he whines about it. A lot. How no one can understand how it feels to be damaged, how his whole identity has been a lie, waaaah, waaaaah, waaaaah. Does he tell Tris any of this? Nooo, because she's "pure" so she can't possibly understand, her genes are awesome and make her perfect. He then makes friends with a GD girl at the compound who's planning a rebellion with the other GD people. Tris finds out he's been plotting with the her, so she blows a gasket when Four asks if she's jealous. Because, of course, perfect Tris is above anything so petty as jealousy. How dare he suggest she'd be subject to human emotions like that. The plan fails and people get hurt, so now Tris is REALLY mad at Four, because she knew that GD bitch was lying all along. Again, she's sooooo not jealous of the girl, just pissed at Four because he's not omniscient, even though she was in on the rebellion too.
Eventually, Tris rejoins the rebellion after finding out the bureau is going to use a memory serum on all the people in Chicago to "reset" the experiment by wiping out their memories. The bureau's obsessed with genetic purity, so they've told everyone that war, poverty, etc. didn't exist before their little boo-boo that ended up damaging people's genes. She "forgives" Four because that's what perfect people do, the forgives Caleb for helping Erudite kill her, breaks into the weapons lab, survives the death serum and releases the memory serum on the bureau to stop their whole "GP's are better" campaign, before being shot by the bureau's leader and dying. It basically turns out that Tris is Jesus (she somehow magically knows when people are lying, the death serum doesn't affect her and she's so selfless, wonderful and otherwise perfect that she sacrifices herself to save her brother and everyone else from having their memory reset). Four saves the city and convinces his mom to be nice, then mopes around forever after he finds out Tris is dead. The GDs and GPs learn to live together peacefully and there are no more experiments. The End.
This installment was so heavy-handed with it's Lesson (i.e. racism, antisemitism, homophobia, pick your irrational reason that one group thinks it's superior to another here, is BAD mmmkay?) that it forgot to actually tell a story. The characters are so flat that you don't care about them. The story is so disjointed that it doesn't really make sense. You move from action scene to action scene with nothing between them to connect the narrative or give any context to the action.
This could have been a really interesting series, but it ended up feeling like the author didn't know where to go with it after the first book was such a hit, so she just threw the second and third books together. Sad and disappointing.