In her senior year as a Southern California college student, girlish sorority president Elle Woods majors in fashion merchandising and is hopelessly in love with her boyfriend, Warner Huntington III, who will attend Harvard Law School the following year. She excitedly expects him to ask her to marry him, but he breaks up with her instead, claiming that he has to be with someone more "serious".
After spending days holed up in her room, Elle's two best friends Margot and Serena take her to get her nails done. While waiting, she finds an article about Warner's older brother and his new fiancee, whom he met in law school. Desperate to win Warner back, Elle takes the Law School Admission Test, applies to Harvard, and is accepted. Upon arriving, her classmates disapprove of her because of her looks and naive behavior, and she discovers that Warner is engaged to another student, Vivian Kensington. The only friend Elle makes is Paulette, a divorced manicurist at a local salon. Elle later helps Paulette gain custody of her dog back from her ex-husband as well as help her seduce the delivery man on whom she has a crush on.
After Vivian tricks Elle into attending a party in a Playboy Bunny costume, Elle has a discussion with Warner and finally realizes he will never respect her. Now determined to succeed on her own, Elle studies hard and wins an internship with Professor Callahan, as do Warner and Vivian. They work with Callahan and an associate, attorney Emmett Richmond, to defend Brooke Taylor-Windham, a famous fitness instructor accused of murdering her much older billionaire husband, Hayworth Windham. Brooke was once Elle's fitness instructor and a member of her sorority. Elle believes Brooke is innocent, but Brooke's stepdaughter, Chutney, and the household cabana boy say she is guilty, and that they saw Brooke standing over Windham's dead body, covered in his blood, while Brooke testifies that she loved her husband and only found him after he had been shot to death.
Brooke refuses to provide Callahan an alibi, but when Elle visits her in prison, Brooke admits that she had liposuction on the day of the murder. Public knowledge of this fact would ruin Brooke's reputation as a fitness instructor, so Elle agrees to keep it secret and refuses to reveal the alibi to Callahan. Impressed by her integrity, Vivian starts to befriend Elle, also admitting that Warner was put on Harvard's wait-list and only got in because his father pulled some strings.
The case against Brooke begins to weaken when Elle deduces that the cabana boy is gay after he correctly identifies Elle's shoe style. During the cross-examination, Emmett tricks him into identifying his boyfriend in court, proving that his testimony about having an affair with Brooke was a lie.
Impressed by her performance, Callahan discusses Elle's future with her and then makes sexual advances on her, which Elle immediately rejects. Overhearing part of the conversation, Vivian is frustrated by Elle apparently using her sexuality to gain her internship. Elle, also thinking that Callahan chose her for sexual reasons, decides to leave law school. Professor Stromwell, who once removed Elle from her class for being unprepared, helps regain Elle's spirit. Meanwhile, Emmett explains Elle's encounter with Callahan to Vivian and Brooke. Brooke is enraged by that and Vivian realizes her mistake. Before the trial continues, Brooke dismisses Callahan and hires Elle as her new attorney with Emmett supervising.
Elle begins shakily while cross-examining Chutney, who testifies that she was home during her father's murder, but did not hear the gunshot because she was in the shower washing her hair after getting her hair permed earlier that day. Elle gets Chutney to reconfirm her story, then reveals that washing permed hair within the first 24 hours would have deactivated the ammonium thioglycolate, and Chutney's curls are still intact. Exposed, Chutney admits to killing Hayworth accidentally because she thought he was Brooke, whom she hated for marrying her father because she was Chutney's age. Brooke is exonerated, and Chutney is arrested. After the trial, Warner tries to reconcile with Elle, but she rejects him, explaining that she needs a boyfriend who is less of a "bonehead" in her new career.
Two years later, Elle, who has graduated with high honors, is the class-elected speaker at the ceremony, and has been invited into one of Boston's best law firms; Vivian is now Elle's best friend and has called off her engagement with Warner, who graduated without honors, no girlfriend, and no job offers; Paulette has married her delivery man and is expecting a baby girl to be named after Elle; and finally, Emmett has started his own practice, is now Elle's boyfriend, and will propose to her that night.