In 1969, young prodigy William Miller (Patrick Fugit) grapples with his identity and seeks to find his place in the world. His situation becomes even more complex when he discovers that his mother, Elaine (Frances McDormand), a strict college professor, has misled him about his age. While William believes he is twelve, the reality is that he is only eleven due to his accelerated schooling, having started first grade at five and skipping fifth grade. Elaine’s rigid constraints against rock music and her paranoia about pop culture deeply affect her children, ultimately pushing his older sister, Anita (Zooey Deschanel), to leave for San Francisco in pursuit of her independence as a flight attendant.
Fast forward to 1973, and William, now fifteen and inspired by Anita's hidden collection of rock vinyl, aspires to break into the realm of rock journalism. He begins contributing articles to underground publications in San Diego. Renowned rock critic Lester Bangs (Philip Seymour Hoffman), the editor of Creem magazine, recognizes William’s talent after receiving his unsolicited writings throughout school, and offers him an assignment to review a Black Sabbath concert for $35. However, to get into the backstage, William must charm his way past the gatekeepers.
When he meets the charismatic lead guitarist Russell Hammond (Billy Crudup) and the free-spirited groupie Penny Lane ([Kate Hudson]), who mentors him on the rock scene, his life starts to change dramatically. Despite the trio behaving like stereotypical groupies, Penny insists on the term "band aids," which she coined to describe female fans who are more passionate about the music than the artists themselves. As Russell develops feelings for Penny, he seeks William's help in creating a connection between them. Penny introduces William to the rock-and-roll lifestyle and brings him along to a hotel where a wild post-concert party is underway, culminating in a surprising encounter for them all.
With his talent recognized, Ben Fong-Torres ([Terry Chen]) from Rolling Stone enlists William to document Stillwater’s tour, offering an enticing $1,000 for a 3,000-word article. As he integrates further into the band’s world, a divide between Russell and lead singer Jeff Bebe ([Jason Lee]) starts to brew, especially when the band's first T-shirt design emphasizes Russell over Jeff. Though William has been playfully dubbed "the enemy" due to his journalist status, he finds it increasingly challenging to maintain his objectivity while becoming part of their tightly-knit group.
However, as the pressure mounts, William is torn between reporting for Rolling Stone and his obligations back home, where his mother is furious about his scholastic neglect. Meanwhile, the band's management troubles escalate when Dennis ([Jimmy Fallon]) is brought in professionally to assist following a disastrous performance, drawing attention to the band's financial pressures that require immediate and more profitable gigs.
And amid it all, the unpredictable road leads Penny to face her downfall, culminating in her being gambled away to another band. The stakes rise on a turbulent flight to New York, where painful secrets emerge, leading to confessions of love and long-standing rivalries coming to a head. The airplane ordeal fundamentally shifts the dynamics among the group.
Arriving back at Rolling Stone, William struggles to finalize his article, reminiscent of Lester's earlier advice, urging him to maintain honesty amidst conflicting loyalties. Their feedback exudes excitement upon reading his article, yet betrayal surfaces when Russell deceives the fact-checkers to safeguard Stillwater’s reputation, causing despair when the piece is ultimately shelved.
In a poignant moment at the airport, William reconnects with his sister Anita, allowing her to drive him home, seeking solace. Not long after, an irate Sapphire confronts Russell over his disloyalty towards William. Gathering the courage to make amends, Russell reaches out to William and agrees to an interview that redeems their relationship. In a fitting twist of fate, Rolling Stone eventually recognizes William's article, showcasing it on their cover, while Penny fulfills her dreams during the band's continued journey.
