On choosing day, then, Beatrice isn't quite sure what to do. Her parents assure her and her brother—who is choosing as well—that they will love them no matter what they pick. Beatrice picks Dauntless and is whisked away to her initation, where she discovers that not everyone who chooses Dauntless gets to be Dauntless. There are a series of training exercises and rankings to tackle, both physical and mental. Those ranked "below the line" will be kicked out and factionless.
Beatrice—who renames herself Tris—initially ranks quite low, especially after a disastrous first fistfight. But she works tirelessly and slowly climbs the ranks, prompted and mentored by Four, a handsome Dauntless with an awesome tattoo. Four seems different than the others, and he and Tris grow closer, especially after the second stage of training begins, in which initiates must overcome their greatest fears in a mental simulation while their trainers watch them do so on screens.
The thing is that Tris overcomes her fears (birds and fire and drowning and such) in a way wholly unlike Dauntless: she beats the simulation by recognizing it isn't real, something a Dauntless would never do. Four realizes Tris isn't ordinary Dauntless at all. She's Divergent—someone whose competency is in multiple areas, someone who can't be neatly categorized and boxed up.
This is a problem because (for mostly vague reasons, though we can guess), Divergents are a threat to the society's leaders and are getting systematically picked off. And as they catch wind that the Erudite are planning to overthrow Abnegation as leaders, led by Jeanine (Kate Winslet), it seems Divergents are in a particularly precarious place.
I should say that I didn't wind up reading the novel before I saw the movie, so I'm not sure whether the film hews closely to its source material. What I can say is that Divergent is a solid movie, far better than most fare for its demographic, and one that most parents and teens could see together and discuss afterwards profitably. But it's not a great film. I can't say for sure whether fans will be pleased. I suspect they will.
That faithfulness to the novel may be what keeps Divergent from being as good as the Other Movie. It focuses so closely on plot (and there is a ton of plot here), and on Tris's transformation into a kickbutt fighting machine, and the romance, that it ignores some of the political and emotional weight it could have had as proper dystopian scifi. (For starters, it's a bit tricky to sympathize with a teenager who apparently does love her parents, but doesn't suffer angst about leaving them behind, apparently forever.)