Twelve-year-old Dawn Wiener is a timid and socially awkward seventh-grader navigating the challenges of adolescence in a middle-class suburban area of New Jersey. She grapples with her insecurities, especially with her older brother, Mark, who is deep into his studies and plays clarinet in a garage band, avoiding relationships with girls to focus on his future. Meanwhile, her younger sister, Missy, is self-indulgent and cunning, often teasing Dawn and prancing around the house in a tutu. Their mother, Marj, is a critical figure who favors Missy in familial disputes, while their father, Harv, is a gentle soul who inevitably aligns with Marj over Dawn.
At school, Dawn faces relentless mockery; her locker is vandalized, and she is unfairly punished when class bully Brandon McCarthy attempts to cheat off her test answers. A further incident occurs when she defensively shoots a spitball at a teacher while being harassed by Brandon and his cronies during an assembly—leading to yet another detention.
Meanwhile, Steve Rodgers, a dashing and charming aspiring rock musician and a friend to her brother, comes into the picture. After Dawn's romantic feelings for him burgeon during their time together, she faces discouragement from one of Steve’s ex-girlfriends, who warns her not to get her hopes up.
After a heated exchange where Dawn calls Brandon a "retard," he retaliates with menacing threats of sexual violence. His initial attempt to attack her fails, but he then calls her, demanding a meeting. When she meets him the following day, he surprisingly engages her in a heartfelt conversation and kisses her. Later that night, a dispute breaks out at dinner regarding her clubhouse, where Marj commands her siblings to destroy it, depriving Dawn of dessert in the process.
Dawn engages in playful moments with Brandon at the clubhouse but reveals her crush on Steve, resulting in Brandon leaving angrily. Heartbroken, she finds Ralphy—the only person who seemed to understand her—trying to soothe her, but she reacts dismissively to his offer of comfort.
At the family’s 20th anniversary party, Dawn plans to approach Steve but falters, leading to a humiliating encounter where Missy shoves her into a kiddie pool while everyone laughs at her misfortune. Overwhelmed by these events, she ultimately destroys the videotape of the party that captures her embarrassing moment and briefly threatens Missy with a hammer while she sleeps.
As the days pass, Brandon faces legal troubles and is suspended from school for alleged drug activities. Complications deepen when Dawn accidentally neglects her little sister, Missy, who ends up kidnapped after a disagreement. Driven by the desire to find her sister, Dawn stumbles across Brandon's home and, after some dramatic exchanges, discovers his tumultuous family life. Despite their connection, Brandon decides he must leave town.
When news reaches Dawn that Missy's tutu was discovered in Times Square, she ventures into the city pursuing any leads to find her sister. After a tense night, she hears from her brother Mark, who informs her that Missy has been rescued unharmed by the police. Upon her return, Dawn faces the scorn of her classmates as she offers a thank-you speech for the support rendered. Mark candidly advises her that she should not expect life to improve until high school, leaving Dawn to contemplate her future as she board a bus to Walt Disney World, joining her choir members in an unenthusiastic rendition of their school anthem.
