Divergent left off. Following the horrifically violent events that the factions just went through, everyone can see that a war is inevitable, and they are all but forced to choose a side.
Some will escape their factions because they disagree with the prevalent attitudes. Some will defect to a new faction out of misguided beliefs (or a simple lust for power.) The factionless will suddenly have more of a voice and a place in society.
And while everyone's beliefs and way of life are changing and being increasingly tested, so is the relationship between Tris and Tobias. Part of me liked the way that they developed as characters, and how those developments made their relationship evolve. But part of me was irritated by it, wasn't always able to accept Tris's decisions.
Once again Roth left us with a fairly shocking cliffhanger, which of course left me with a fairly strong desire to rush out and find out what happens next. But it also left me with a bit of an emotional hangover, so I'm going to have to wait a little while (and read something a little lighter for the palate cleansing effect) before I can come back and finish the series.
For me, this book was slightly slower to get into than the first one. (It took me almost a week to read, rather than just over a day.) But I still really enjoyed it, and the payoff was totally worth it.