Angela Delvecchio (Kellie Martin) is a high school sophomore in the small northern California town of Santa Mira who aspires to be popular, beautiful and perfect in everything, much like the school's "queen bee" Stacy Lockwood (Tori Spelling). She idolizes Stacy, who is the most popular girl at school, and wants to become a cheerleader. When Angela is accepted the "Larks," the school's popular clique, she tries to forge a friendship with the snobbish and conceited Stacy, who rejects her.
Things only get worse for Angela, who suffers the double whammy of having been rejected for a coveted position on the yearbook staff and not making the cheerleading squad as the school year draws to a close. Still determined to be friends with Stacy, she calls Stacy's mother one evening and anonymously invites her to a party under the guise of there being a special dinner for the Larks. Once Stacy gets in the car with Angela, she reveals that the "dinner" story was a lie she told Stacy's mother so she would be allowed to come, and after they drive some distance, it is revealed that Angela was not directly invited to the party herself.
Infuriated, Stacy demands to be taken home, but Angela tries to explain how much she admires Stacy and wants to be like her. Unmoved, she calls Angela "pathetic" and gets out of the car. She gets a ride home, but Angela follows her. Worried Stacy will spread rumors about her, Angela attacks Stacy in front of her house with a knife and stabs her multiple times and leaves her for dead.
Angela avoids capture in the weeks following the incident, as another student, an outcast Goth girl named Monica, is blamed for Stacy's murder - no one suspects Angela due to the fact she is seemingly too nice to commit a crime - and also because one of Stacy's friends, named Jamie, tells Angela that no one really liked Stacy, but that they were only afraid of her.
As her junior year begins, Angela becomes more involved with the community, taking up such activities as peer counseling and candy striping. However, overwhelmed by Stacy's murder, one of the Larks brings up the idea of disbanding. Determined not to let this happen, Angela argues that they should remain active, noting that the group was not only important to Stacy, but also to the various community activities in which they take part. This idea not only saves the Larks, but also wins Angela the position of secretary/treasurer.
In the meantime, a harassment campaign is waged against Monica until she finally leaves the school. At this point, authorities resume their investigation and begin re-interviewing possible suspects, including Angela. With the authorities slowly closing in on her, she becomes more and more consumed by her guilt, until she finally confesses to her parents in a letter.
Devastated by Angela's arrest, Jamie confesses to having left Angela in a ski lodge alone during a ski trip the year before, all because she did not have the courage to stand up to Stacy. As the trial begins, the prosecutor claims it was premeditation and first-degree murder, but Angela's lawyer claims it was second-degree murder.
The judge agrees with the prosecutor, after listening to Angela's taped confession that Angela killed Stacy fearing public humiliation. Angela is then sentenced to confinement until the age of 25. As the movie ends, Jamie writes a letter to the imprisoned Angela that she quit the Larks (having left when she realized how mean they were to Angela) and that she plans to quit Santa Mira High and go back to her former Catholic high school, St. Josephs.