So good, in fact, that it almost defies description. I must not be the only one who feels this way, either, because most of the reviews and blurbs I've read actually saying almost nothing about what happens in the book. Without giving away too much, here's my feeble attempt:
Insurgent begins barely a breath after the final scene in Divergent. After destroying the simulation program that caused the Dauntless to brutally attack Abnegation, Tris and Four take refuge with the Amity, along with Marcus, Peter, Caleb, and a handful of Abnegation refugees. But they can only stay as long as they can abide by Amity's code of behavior, and after killing one of her best friends and watching her parents get shot to death, Tris has no aptitude for a world of happiness and harmony. The Erudite aren't particularly interested in honoring Amity's stance as a neutral faction anyway. Before long, they are on the run again. Finding the other Dauntless who have refused to join the Erudite and Dauntless traitors, Tris and Four face the grim possibility that if no faction is left to help them, they may have no other option but to join the factionless.
This is more or less the premise of the book, but there's a lot more to it than just Tris and Four running around the streets of dystopian Chicago. The first book introduces a love story that I can't help but call precious even though the characters are both fierce and fearless. The second book puts that love to the test. Four has always pushed Tris to build her courage, to know her own strength. He has never let her back down or give in to fear. But when her courage turns daredevil, he knows the difference. Their relationship is a reminder that we should be the kinds of people who expect the best from those we love, forgiving weakness but not enabling it. We should be those sorts of people for others, and we should look for those sorts of people for ourselves.
It's hard to talk about Tris without talking about her strength, but Tris isn't interesting because she is strong. She's interesting because she becomes strong, and her strength is more than just her bravery and battle skills. If this book teaches us anything, it's that there's virtue in striving for virtue. In a sense, you might think the book is saying quite the opposite - these corrupt factions, after all, are the result of a group of people deciding to pursue a single virtue exclusive of all others. But Tris is Divergent, and we see the world through her eyes. She will always be striving toward more than one virtue, always leaning toward a better way, always finding what is right and what is hurtful in each of the factions. On the one hand, this makes her self-critical to the point of self-destruction. On the other hand, it also makes her a heroine.
When I was a teenager, I was single-minded about bettering myself - whether it be in school or just generally as a human being. I was very aware of different virtues that I had and that I didn't have, as though they were clearly demarcated factions within myself. It's been a long time since I've felt that way, and I know that in a sense I'm healthier for the change. But in another sense, I have lost something, too. I have not forgotten, but I have certainly ignored the fact that I can change for the better - and that I should. I am nothing like Tris, and my world is radically different than hers. But she is a reminder of what I have been and what I ought to be.