The plot of the actual crime is not the brightest but that's the only weak point of this movie.
The directing and photography are really, really good, with an atmosphere that remembers to the latest rising of the Scandinavian crime novels. Stieg Larsson, and series like Bron/Broen (which have been later deeply inspiring US series like "True Detective") are very present in this movie.
The cast is superb and gives us great performances. Arévalo and Gutiérrez, which usually perform comedy roles, are fantastic, as also are Barros and Reina. The dialogs are really good and also the several loose ends that the story leaves.
To finish, the movie moves around the history of the under-dictatorship and incipient democratic Spain in an indirect way used quite often in Spanish movies. This lets the viewer the door ajar so they can be curious enough to enter and look for more information on their own, while leaving the movie in a safe spot, far from the criticism that could suffer from those in Spain still not acknowledging the darkest period of the recent history of the country.