First, I have to say that I am a Cuban, living here in Havana. Several of the scenes regarding the murder of the young teacher were shot a few blocks away from my parent's home.
If you really wanna understand completely the plot(s) you have to read the books by Leonardo Padura. Better still if you can read those directly in Spanish.
I live in a very rough neighborhood, accurately depicted in the series. The characters are quite realistic; they act, talk, think just as described. Cuban police is very much alike the police forces in the series. (Even when the headquarters is not an actual police station, but a beachside resort, ahahaahaha)
I've read an user complaining about weak police drama; I have to disagree. First, murder in Cuba, even in Havana, is not a very common deed. Homicides occur, but usually with close to none planning, as a result of a quarry, over women, or crime- related account settings. This user also says that crooks usually confess without needing to; but then you have to understand the way we Cubans think about crime and punishment. Usually when somebody does something very wrong, the bet is on not being caught, on remaining clear of the police. But once caught, almost everybody thinks that the game is over (and it usually is). Cuban police does not use torture, in the Abu-Graib way, but they have a lot of very capable officials pushing on every button of the suspect's mind to obtain a confession. So, the series is also realistic in the relation between crooks and police. Police forces are depicted the way they are: a non monolithic organization, where you are going to find good, not so good, and frankly bad, persons, with their real lives, conflicts, mistakes, struggling to do the good the best way they can find.
Again, my advise, read the books by Padura