The film begins with Leo Borlock (Graham Verchere), a 14-year-old boy in a marching band uniform, practicing on a sun-drenched football field. Out of nowhere, a mystical girl, Stargirl Caraway (Grace VanderWaal), appears like a mirage, captivating and unsettling him. However, before Leo can approach her, he gets lost in his memories, reflecting on his childhood.
Having faced the heart-wrenching loss of his father, Leo often wears a porcupine tie, a nostalgic emblem of remembrance. He and his mother, Gloria (Darby Stanchfield), relocated to Mica, Arizona, a place where time seems to pause. Year after year on his birthday, Leo receives a new porcupine tie, gifts steeped in mystery that initially drew ridicule from his peers but ultimately became a part of his identity.
In his current life, Leo enjoys the company of close friends Kevin Singh (Karan Brar), Tess Reid (Annacheska Brown), and Benny (Collin Blackford), as they run a school radio show called "The Hot Seat." Amidst this routine, Leo finds himself irresistibly drawn to Stargirl, enchanted by her ukulele tunes. She serenades him on his birthday, leaving him awestruck.
As the Mudfrogs football team competes against their rivals, Stargirl shines brightly, infusing the game with her energetic ukulele melodies that invigorate the players and lead them to victory. With each triumph, Stargirl’s popularity surges, transforming her into a beacon of good luck for the team.
To dig deeper into Stargirl’s story, Leo approaches Archie (Giancarlo Esposito), a local paleontologist. Archie divulges precious insights about Stargirl's upbringing as a homeschooled child, raised by her single mother, who often sought his guidance. As Leo becomes increasingly enchanted by Stargirl, Archie playfully warns him to be cautious of her captivating charm.
During an upcoming radio show, Kevin invites Stargirl to join them, but she turns down the offer playfully. Torn between his curiosity and the tension brewing among his friends, Leo embarks on a journey of self-discovery intertwined with Stargirl’s enigmatic presence. This sets the stage for a cinematic adventure where reality and fantasy mingle, revealing the extraordinary essence of human connection.
The Mudfrogs achieve yet another victory at their next game, courtesy of Stargirl’s spirited rendition of "We Got The Beat." In the meantime, she quietly enters the esteemed school speech competition, leading to friction with Kevin. As she continues to build her relationship with Leo, they share profound moments, including a scenic view of the Grand Canyon, where Stargirl encourages Leo to embrace his true feelings and express his frustrations through heartfelt screams.
As the speech competition unfolds, Stargirl mesmerizes everyone with her speech about rats. Later, she surprises Leo by visiting a young boy recovering from an accident, which leads Leo to realize she has been secretly gifting him porcupine ties for his birthdays—a gesture she humorously denies. On a visit to her home, where Leo meets her mother, Ana (Sara Arrington), he learns more about Stargirl's world as she introduces him to her prized record collection and recommends the song "Thirteen" by Big Star. Their evening culminates in a tender kiss, signaling the blossoming of their romance.
However, drama ensues when a player from the opposing team, Ron Kovac (Troy Brookins), get injured in a match, prompting Stargirl to leap into action and help him, leaving everyone shocked and bewildered. Leo confronts her about the resultant loss of the game but she stands firm in her conviction, saying no one else had stepped in to help. As their connection flourishes, the challenges they face grow proportionately.
The students’ disdain for Stargirl’s well-meaning actions swells, especially after her appearance on "Hot Seat," which further alienates her from the group. The climax occurs when Hillary Kimble (Shelby Simmons) reveals a painful truth: the injured boy was her brother, leading to a dramatic fallout that sends Stargirl fleeing in tears.
Following a heart-wrenching confrontation with Leo, she realizes he's urging conformity, which deepens her emotional turmoil. The impact of Stargirl’s presence continues to linger, leaving everyone pondering the enigma that she embodies.
Upon returning to school as Susan (formerly Stargirl), she finds her place among her peers again, yet struggles with her authenticity. When participating in the speech competition, she initially opts for a safe topic but ultimately delivers a powerful and unique performance as Stargirl, winning the competition. With her trophy in hand, she and Leo proudly display it in their school’s trophy case.
Yet, even with this accomplishment, Susan feels the pressure of societal expectations crashing upon her. When Leo asks her to the Winter Dance, she declines, burdening him with guilt. However, encouraged by his mother and Kevin, he reflects on her altruistic nature.
The dance night arrives, and to Leo’s delight, Susan attends, revealing that the band invites him to perform "Just What I Needed." The energy in the room surges as the students collectively join in. But as the music fades, Susan mysteriously vanishes into the night, leaving Leo to ponder over her disappearance and its implications.
Days later, Leo encounters clues left by Susan: records, a vintage player, and cryptic notes that highlight her legacy of kindness and her quest to bring joy to others. Upon finding her former hideout at Archie’s, Leo discovers more about her desires to uplift those around her.
Standing at the Grand Canyon’s edge, he channels his pent-up emotions just as Susan had encouraged him to, culminating in a poignant moment that echoes her lasting influence. As the credits roll, viewers witness the overflowing trophy case with Stargirl’s trophy proudly on display, symbolizing an unforgettable imprint on the lives she touched. In a heartfelt finale, Susan’s gentle song lingers, forever etching their bond and shared memories into the hearts of those who encountered her magic.
