It's San Diego, 1973, and 15-year-old William Miller (PATRICK FUGIT) is a huge music fan, thanks to the collection of albums his older sister, Anita (ZOOEY DESCHANEL), left him when she suddenly left home several years earlier to be a stewardess. A budding music journalist, William eventually meets Lester Bangs (PHILIP SEYMOUR HOFFMAN), a legendary critic who warns William about the death of rock and roll and the lengths to which bands and promoters will go to in making sure that critics and reporters are happy so that they'll write positive stories about them and their music.
Although reluctant, Lester likes William's enthusiasm and thus assigns him to do a story on Black Sabbath. William's mother, Elaine (FRANCES McDORMAND), a college professor and restrictive parent, isn't crazy about the idea, but drops him off at the concert, making him promise that he won't do drugs.
Once there, William meets several groupies, including Sapphire (FAIRUZA BALK), Polexia (ANNA PAQUIN) and Penny Lane (KATE HUDSON), the latter of whom he's instantly smitten with. He also meets the up and coming band, Stillwater, consisting of lead guitarist Russell Hammond (BILLY CRUDUP), lead singer Jeff Bebe (JASON LEE), bass player Larry Fellows (MARK KOZELEK), drummer Ed Vallencourt (JOHN FEDEVICH) and their manager Dick Roswell (NOAH TAYLOR). As they're opening for Sabbath, they manage to get him backstage for their performance.
Although they consider him the enemy since they play for the fans and not the critics, the members of Stillwater like William's style and thus invite him along on their tour. With his mother reluctantly letting him go, and then getting a freelance assignment to write a story for Rolling Stone magazine, William travels across America with the band and witnesses the varied dynamics and dramatic conflict of the rock and roll lifestyle while on the road.