Humanity has been divided into 5 groups to keep the peace. And the selfless, the Altruan, so to speak, form the government. To this fraction heard Beatrice Prior, at least until 16 assumes the determination ceremony in part, because there you have the chance to opt for a test for a different group. This system is somewhere already pretty crass, because the motto fraction of blood. And Beatrice decides, like her brother, for another group. When it comes to the Tris Fighter Group, the Ferox, where she meets and Four, their instructor.
In the book it comes to these fractions has any properties, such as tris makes itself as initiator in the Ferox, as well as a conspiracy. Unlike the book, I think, Tris acts lame in the film. It is not so much to respond to its problems with the other founders, or their internal struggle. Also with the indefinite is not as nice explains how to mention in the book, Four completely. I will of course not spoilers, but if you like the movie, you will love the book. And like I make me ran to the second part.