Imagine a society where everyone is divided into five factions, based on what they value most: Candor (honestly), Abnegation (selflessness), Dauntless (bravery), Amity (peace) and Erudite (intelligence).
Each year a Choosing Ceremoney is held where all sixteen-year-olds must choose which faction they will live in. Beatrice, who grew up in Abnegation, must choose between staying with her family, or being true to herself. But soon she is caught up in something much bigger, and must make life-or-death decisions affecting her and everyone she's known.
A common criticism of this book by other reviewers is that it is unrealistic that everyone would fit into five virtues. Although I agree with this, it is clear in the book that this society is not working, in part for that very reason.
What I found more difficult to suspend my disbelief about was Beatrice's incredible amount of "girl power", given her background (I could not help but keep thinking "she is no Katniss Everdeen" while I read). I also didn't think her reason for her choice of factions seemed very convincing (other than as a plot device by the author to lead the book to its inevitable conclusion). I think this could have been solved by a prologue showing an incident from her childhood that foreshadowed her behaviour.
However "Divergent" is such a riveting read, I was more than happy to overlook its flaws. So that is why I give this book four out of five flying bird tattoos, and look forward to reading "Insurgent" in May.

*Spoilers" Don't click "Read More" below until you've read the book.