As a passionate metalhead I was excited to watch a movie on the subject. It has some cool interviews and the documentary tries to map the metal's evolution from the hard rock beginnings in the 60s to the multitude of subgenres today. It does well with heavy and thrash metal but when it comes to the extremes, it barely touches the surface of styles like the norwegian black metal, and omits important styles such as doom metal, or black metal with folk roots, or "alternative" projects that combine five types of male & female vocals with black metal guitars, heavy metal riffing, flutes, violins, techno/trance rhytms and ambient landscapes all in one song... Metal undeground is very complex and the variety is immense - so it is understandable that fitting all this into 90 minutes borders with the impossible. Yet it is obvious that the author is more knowledgeable about heavy metal than the recent developments in the most extreme realms ever known to the music world. Overall, quite enjoyable!