a a passionate metalhead i be excite to watch a movie on the subject . it have some cool interview and the documentary try to map the metal evolution from the hard rock beginning in the 60 to the multitude of subgenres today . it do well with heavy and thrash metal but when it come to the extreme and it barely touch the surface of style like the norwegian black metal and and omits important style such a doom metal and or black metal with folk root and or alternative project that combine five type of male and female vocal with black metal guitar and heavy metal riff and flute and violin and techno or trance rhytms and ambient landscape all in one song . metal undeground be very complex and the variety be immense so it be understandable that fit all this into 90 minute border with the impossible . yet it be obvious that the author be more knowledgeable about heavy metal than the recent development in the most extreme realm ever know to the music world . overall and quite enjoyable .