The film discusses the traits and originators of some of metal's many subgenres, including the New Wave of British Heavy Metal, power metal, Nu metal, glam metal, thrash metal, black metal, and death metal. Dunn uses a family-tree-type flowchart to document some of the most popular metal subgenres. The film also explores various aspects of heavy metal culture.

Although he was only 25 when he died Michael Reeves opened up genre cinema to new possibilities. Now considered one of the forefathers of folk horror for his groundbreaking masterpiece Witchfinder General (1968) Reeves certainly left his mark as a director. The fact he wasn't around long enough to expand on his legacy, let alone celebrate it, is one of the biggest tragedies of British film history.