In 1969, child prodigy William Miller struggles to fit in with the world. His widowed mother Elaine   who has led him to believe he is 12 years old, until Williams older sister, Anita, tells their mother to tell the truth, his age is actually 11, from being placed into First Grade at the age of 5, and skipped 5th grade. Their mother strictly controls and protects him and his older sister Anita, forbidding rock music and other unwelcome influences, driving Anita to leave home and become a flight attendant.
In 1973, 15-year-old William, influenced by the rock albums left by his sister, aspires to be a rock journalist, writing freelance articles for underground papers in San Diego. Rock journalist Lester Bangs, impressed with his writing, gives him a $35 assignment to review a Black Sabbath concert. He can't get backstage, but opening band Stillwater arrives, and after he flatters them with critical praise they bring him along. Lead guitarist Russell Hammond takes a liking to him, partly because of William's new friendship with veteran groupie Penny Lane, though she prefers the term Band-Aid. Feigning retirement from her glory days, she takes William under her wing, but only to get close to Russell, for whom she has genuine feelings.
William is contacted by Ben Fong-Torres, editor of Rolling Stone, who believes him to be older and hires him to write a story. He convinces Ben to let him write about Stillwater, and he is instructed to go on the road with them. Tensions between Russell and lead singer Jeff Bebe are evident. William begins to interview the members of the band, but Russell repeatedly puts it off. Penny watches the interaction and sympathizes with William, whom they joking call "the enemy" because he's a journalist, but he quickly becomes part of their inner circle as he loses his objectivity.
The band experiences problems with promoters and venues on the tour, and hires Dennis, a professional manager. Penny is told she must leave them before New York, where Russell's ex-wife/girlfriend Leslie will join them. Stillwater "loses" Penny's three prot g  groupies to another band in a poker game; she acts nonchalant but is devastated. Also, Dennis has chartered a small plane to allow them to play more gigs than their tour bus does, and there is no room on it for Penny, who is left behind.
However, Penny goes to New York on her own, and shows up at the restaurant where the band is celebrating the news that they will be featured on the cover of Rolling Stone. Leslie notices her apparent attempts to get Russell's attention, and Penny is asked to leave. William chases her back to her hotel, where he saves her from overdosing on quaaludes.
The next day, the band flies to another gig, but the plane encounters severe weather. Believing they are about to die, members of the group confess their secrets to one another, bringing the conflicts between Jeff and Russell into the open. Jeff insults Penny, but William defends her and confesses that he loves her. The plane lands safely, leaving everyone to ponder the changed atmosphere.
William leaves the group, to finish the article at the Rolling Stone office in San Francisco, writing the whole truth about what he has observed. Fearful the story will damage the band's image, Russell tells the magazine's fact-checker that the boy's story is untrue, killing the article and crushing William emotionally. William's sister happens to encounter him sitting dejected in the airport, and offers to take him anywhere in the world; he chooses to go home to San Diego, where their mother is overcome to have them back.
One of the groupies chastises Russell for what he did to William. Russell calls Penny and asks to meet with her, but she tricks him, giving him William's home address instead. He is forced to face William and apologizes; William finally gets his interview, and Russell reverses himself and confirms William's article to Rolling Stone, which runs it as a cover feature. Meanwhile, Penny purchases a ticket to Morocco, fulfilling her long-standing fantasy.
Output: What concert does Lester Bangs want William to review?