Dawn Wiener (Heather Matarazzo) is a shy, unattractive, unpopular 12-year-old 7th grader in a middle-class suburban community in New Jersey. Her older brother Mark (Matthew Faber) is a nerdy 11th grade high school student who plays the clarinet in a garage band and shuns girls in order to prepare for college. Dawn's younger sister Missy (Daria Kalinina) is a seven-year-old spoiled, manipulative little girl who pesters Dawn and dances happily in a tutu. Their mother (Angela Pietropinto) is a shrewish, overbearing woman who dotes on Missy and always sides with her in disputes with Dawn. Their father is a meek, selfish businessman who always sides with Dawn's mother. Dawn's only friend is an effeminate fifth-grade boy named Ralphy (Dimitri DeFresco), with whom she shares a dilapidated clubhouse in her backyard.
Dawn's life in junior high is even more miserable: all of classmates call her names and cover her locker with derisive graffiti, the cheerleaders call her a lesbian, an abrasive teacher unfairly keeps her after school one day for no reason, another girl forces her to use the toilet while the girl watches, and she is threatened with rape by a bully named Brandon McCarthy (Brendon Sexton), who has almost as much trouble socializing as she does. Her attempts to take out her frustrations only get her into trouble.
At home, her mother punishes her for calling Missy a lesbian and refusing to be nice to her (considering that Missy is never friendly towards Dawn). The next day at school, Dawn accidentally hits a teacher in the eye with a spitball and she gets suspended for three days. Dawn's stupid and ignorant parents continue to be unsure of what is causing her rude behavior and Dawn refuses to talk to them about her hellish life at school, aware that they will not understand.
Brandon's first attempt to rape Dawn after school fails when he does not seem to force himself upon her, but he orders her to meet him again the next day after school. After she presents herself to him and he takes her to an abandoned field, he starts an earnest conversation and only kisses her, even though she is obviously willing. Brandon is clearly not the bad seed that he puts himself out to be, but just like Dawn, he is a socially awkward kid who hides his own unhappiness behind anger and aggression.
Meanwhile, Mark's band is joined by Steve Rodgers (Eric Mabius), a cool, handsome 19-year-old guy who intends to become a rock star. Dawn immediately falls in love with him, and she decides to pursue him romantically after he spends some time with her, even though one of Steve's former girlfriends explains to Dawn that she has no chance to become his current girlfriend.
As the film continues, Dawn and Brandon treat each other ambivalently. He bonds with her in her clubhouse only after another girl rejects him, and Dawn pursues him only after Steve rejects her. Toward the end of the film, Brandon is expelled from school for suspected drug dealing and, after kissing Dawn, runs away to avoid being sent to military school. Dawn comes over to Brandon's house the day he runs away to visit and sees that his home life is even worse then hers; He has an abusive and alcoholic widowed father, and a mentally retarded older brother who requires constant supervision. After angrily rejecting Ralphy as a boyfriend, Dawn is left with no friends at all.
The next occasion of Dawn's embarrassment is a lawn party to celebrate her parents' 18th wedding anniversary. When Dawn refuses to tear down her clubhouse to make room for the party, her mother has Mark and Missy destroy it and gives them her share of a chocolate cake. At the party, Missy humiliates Dawn when she pushes her into a kiddie pool for no reason in front of the assembled guests who laugh at it. That evening, the family watches a videotape of the party, with all of them (except Dawn) laughing when Dawn falls into the water, which further alienates her as her immature parents continue to remain oblivious to Dawn's alienation. Later that night, Dawn smashes the videotape to pieces with a hammer and briefly brandishes her hammer over Missy's bed as she sleeps to bludgeon her to death. But Dawn cannot bring herself to commit murder and instead walks away and goes to bed.
Dawn's ultimate personal disaster occurs a few weeks later when her father's car breaks down while he is driving home from work and her mother has to pick him up. Dawn is to tell Missy to find a ride home from ballet class while their mother is away. However, Missy argues with Dawn, who retaliates by not telling her, with the result that Missy is kidnapped by a stranger while she is walking home alone. When Missy's tutu is found in Times Square, Dawn goes to New York City hoping to find Missy and become a hero to make her difficult parents notice and respect her. After a full day and night of searching fruitlessly for Missy, Dawn phones home to check up on what is going on and Mark tells her that Missy has been found by the police after she was abducted by a pedophile neighbor. Dawn returns home dejected and sees that her mother was too preoccupied with Missy's traumatic kidnapping ordeal to even notice Dawn's absence. But at least to Dawn's credit, her parents never figure out that she was indirectly responsible for Missy kidnapping by withholding information from her that led to Missy being abducted in the first place.
After this, Dawn's classmates make fun of her as she gives a speech in the assembly hall to thank them for their "emotional support". It is only after the principal orders the unruly students to be quiet, Dawn gives herself enough mental strength to finish her speech.
A few more weeks later, school is let out for the summer. Dawn is relieved that her painful ordeal at school is over... at least for the next three months. Afterwards, Dawn's mental state takes a further blow when Mark offers her no hope of a better life until high school... which according to him is will not be much better.
As the movie comes to an end, Dawn finds herself literally back where she started... only worse. She is still unpopular and unattractive. Her parents have gone from mistreating and neglecting her to ignoring her completely in favor for Missy's abduction. Rather then counting down the long summer days to go back to school for 8th grade (and thus endure another hellish year of bullying and abuse), Dawn decides to get away from her home life by signing herself up for a month-long school summer trip to Florida's Walt Disney World. Her self-involved parents are still too pre-occupied with their own lives to approve of Dawn's permission to leave for the summer, and do not even notice her packing up to leave, much less say "goodbye" to her.
The final scene shows Dawn sitting anonymously among her other classmates on the school bus as she obediently joins other students singing the school anthem during the field trip to Florida as she sorrowfully looks out the bus window to ponder her uncertain future.