Some time in the future, there was a disaster. We aren't given the details in Divergent. Roth drops us right into the middle of things, as Tris and her brother have their aptitude tests to determine which of the five tribes they would best fit into. Tris grew up in Abnegation, the tribe that devotes itself to selflessness and courtesy towards others. But she's never felt like she belonged. Being completely selfless was always a struggle for her. When the time comes to choose, she joins Dauntless because she feels a sense of kinship towards their quest for bravery. The training period is a struggle. Failing means joining the outcast factionless and a short, miserable, hungry life. Tris survives and thrives in her new tribe. Everything would be perfect if it weren't for the fact that Tris is Divergent; she has an aptitude for three tribes, not just one like everyone else. And, for some reason, if Tris is found out, she will be killed quietly, so as not to upset the status quo. All hell breaks loose on the day Tris graduates. A plot, hatched by the leader of the Erudite (who prize knowledge), causes non-Divergent Dauntless to become mind-controlled automatons who then attack the Abnegation leadership. Divergent ends with Tris and her friends and allies seeking refuge with Amity (peace-seekers).
As I read Divergent, I wondered how this society had come to be. The system was so strange and artificial that it could not have evolved naturally. Five different tribes, each devoting themselves to just one quality and actively suppressing others, will inevitably lead to conflict. It's not a stable system. And this flies right in the face of what Tris has been told about her city's origins—that they withdrew because they wanted to find a way to live in harmony through their various paths. Calling the five groups religions is as fitting as calling them tribes. The system sounds more like an elaborate thought experiment than anything else. At the end of Insurgent, I found out that my suspicions were right. It is artificial. It was a thought experiment played out in real life. And I'm going to have to want (impatiently) to see how it all plays out in part three to see what happens when Tris and her comrades venture outside of their city.
One of the defining characteristics of young adult fiction is how didactic it is. The genre is often as much about self-discovery as it is about entertainment. That especially apparent in this series. Almost every plot point and character can be read as a metaphor for something without much effort. That not a bad thing, as Divergent and Insurgent are fairly well written. I would say very well written but Roth is not a subtle writer and several of the characters undergo emotional whiplash as they're forced to do what the plot needs them to do.
It's still a fascinating story, in spite of its problems. I can forgive problems that spring from the author's purpose rather than from an author's lack of ability (e.g. the Twilight series). Besides, the main character of this series is a strong girl, who is conflicted but still heroic.