In the early 1980s, a cluster of teenage bands in the San Francisco Bay Area turned garages and small clubs into the birthplace of thrash metal. The film traces the scene from Exodus and Metallica's early Bay Area gigs through the rise of Slayer, Testament, and Death Angel, using interviews with the musicians themselves alongside photographers Harald Oimoen and Brian Lew, whose book the film is built on. Concert footage from clubs like the Stone and the Old Waldorf sits next to home movies and flyers that show how word of mouth and tape trading built an audience before MTV or radio noticed. Members of Metallica, Slayer, Anthrax, and Megadeth describe the friendships and rivalries that pushed the sound faster and heavier, and the film credits the scene's DIY ethic, kids booking their own shows and dubbing their own cassettes, for turning a regional curiosity into a genre that reshaped heavy metal worldwide.