This is like a lot of movies where the premise is more interesting than the plot. The arrival of the "Newcomers" seems to be a metaphor for the immigrant experience in America, but this is not followed up. It goes from a sci-fi movie into a cop-buddy movie, and it's really not bad on that level. It seems, however that the filmmakers passed on an opportunity to make a really significant commentary. One wishes for the film that got away.
Review of Alien Nation
