This is the adaptation of the first book of a trilogy. I think that people who have not read the books will have some trouble to follow the film. It is a very weak and dissapointing adapation which leaves outside some of the best things of the books: for example, the grandfather, one of the greatest carachters, only appears in one or two very brief scenes. The relation between Unai and Alba does not translate well to the screen. The rich traditional background you have in the book comes very superficial here, and so happens with the investigation itself. Also, having the "big mistery" revealed 20 minutes into the film does not help.
Generally all (acting, music, script...) is quite forgettable and superficial and lacks the magic the books have. In my opinion, it could (and should) have been made into a TV series, not into a movie.