Beatrice and her brother Caleb have turned sixteen, which means it is now time to decide which faction they will choose to be a part of: Candor, Abnegation, Dauntless, Amity, or Erudite. Having both grown up as Abnegation, it is assumed that at least one of them will choose to remain. Once they have chosen, there is no going back.
Beatrice "Tris" discovers that there's something different about her and is warned not to discuss these differences with anyone. She soon finds herself part of a world where she must fight-physically and mentally, for a position among the top 10, or face the possibility of being factionless.
It is difficult to continue without giving away too much detail. Just trust me, as the story unfolds, it just keeps getting better and as the end of the book approaches you'll want to read slower to make it last as long as possible.