This movie tells how religion can be used to lead people to fanatism.
By extremist which is the purpose this film want to demonstrate, but not only it could be anyone with hidden motive (and actually preacher in "hidden" mosque are compared to sect leader by the way they brainwash their followers).
We follow a journalist, that intends to write a book on extremist islam in France.
The comeback from Afghanistan from one guy transforms it in the making of an jihad group in Paris, and the vicious circle is triggered.
The point of view chosen, of this journalist, is interesting as it creates an objective distance with the ideas, the easiness everything progress towards the worst. It also shows how someone that do not share these ideas can still be affected, only because of the links created with others.
And of course, it adds a tension in the movie wether or not he will be uncovered.
The realistic treatment given to the initial situation and story adds to that and makes it a really interesting movie, that is worth many "headline investigations" that water on TV, especially the first part.
Of course, not everything is realistic, some characters are'nt that convincing (and tries too much to handle every kind of possible jihadist) and their development can be too quick. The second part of the movie is more evolving into a genre movie but keeps things at a realistic level.
It tells above all of the easiness with which lost lambs can fall into fanatism traps.