Let me start by saying that this film impressed me by the mere fact that it repeatedly broke rules of logic and I didn't care. Usually I would be grumping that such-and-such didn't make sense, but this film was such fun to watch, and established so early on that it didn't care about the rules, that I just enjoyed myself instead. I haven't seen the first movie, Plaga Zombie, but it seems that Plaga Zombie: Zona Mutante picks up where the last left off. The three heroes from the last film, pro wrestler John West (Berta Muñiz), medical student Bill Johnson (Pablo Parés), and computer nerd Max Giggs (Hernán Sáez), are rescued by the FBI, but then, because they now know what's going on, the FBI decides to dump them right back into the zombie-infested town they were saved from. The FBI is partly responsible for the outbreak in the first place, because they agreed to let aliens use the town as a testing ground for an alien virus. Those that have seen the first film may be wondering about a major continuity problem at this point, but let me just say that it all gets explained. More or less.
This film was also written by Parés and Sáez, and their style reminds me a lot of how Raimi started out in The Evil Dead, taking a good script and a cheap budget, and covering his stars in gore. I don't think any of the stars spends more than two minutes of screen time with a clean face. They also share Raimi's sense of the absurd (including borrowing gags from The Three Stooges), so fans of the Evil Dead series should definitely see this film.
The gore is over the top. Come to think of it, it reminds me a little of Braindead, so there's another gem to compare this movie to. The effects are inventive and disgusting, so gore fans will not be disappointed. On the other hand, the movie isn't scary in the least, but the pace never flags as the heroes run about the town, trying to find a way out.
The movie did have its weak points. There were some parts that puzzled me that I never did figure out, not just logical loopholes but plot gaps. Maybe I would have understood if I had seen the first movie. Nevetheless, on the whole I was very satisified.