Elle Woods, a student of fashion merchandising and president of her sorority, is taken to an expensive restaurant by her boyfriend, Warner Huntington III, who is the governor's son. Expecting Warner to propose, to her dismay, he breaks up with her, thinking that she is not serious enough for his political aspirations. Elle seeks comfort from her sorority sisters, who encourage her to keep pursuing him, leading to her plan to follow Warner into Harvard Law School. After much effort, she scores a 179 on the Law School Admission Test, and combined with her 4.0 GPA in fashion merchandising, the Harvard panel admits her.
Upon arriving at Harvard, Elle's SoCal personality is a contrast to her arrogant, drab, East Coast classmates, who refuse to take her seriously. Elle soon encounters Warner, but finds out that he is engaged to another classmate, Vivian Kensington. To make things worse, her studies are off to a poor start, as she struggles to keep up with the reading, and shows up unprepared for a lecture, causing a strict female professor, Professor Stromwell, to heavily berate her. After being kicked out of the class, Elle meets Emmett, who gives her advice for handling each of her professors. Vivian invites Elle to a costume party, and Elle shows up in a playboy bunny suit, when it is actually not a costume party. Elle tells Warner that she intends to apply for one of her professor's internships, but Warner tells her that she won't be accepted and that she's just wasting her time. Elle realizes that Warner will never take her back. She finds motivation to prove herself. Determined to succeed against the odds, Elle studies diligently and eventually receives an internship at the legal office of Callahan, the school's most respected professor, who is defending a prominent fitness instructor named Brooke Windham. Accused of murdering her husband, Brooke is unwilling to produce an alibi. The prosecution's main witness, a pool cleaner, claims that he was having an affair with Brooke, and that the two were going to run off with Brooke's husband's money. In fact, Brooke reveals to Elle that she was having liposuction during the murder, but forces her to keep the alibi a secret as it would ruin her career and reputation. Impressed by Elle's ingenuity, Vivian befriends her and admits to certain things about Warner that even Elle never knew about.
Elle, using her fashion sense, deduces that the pool cleaner is gay, but Callahan dismisses her theory. However, Emmett, Callahan's junior partner, believes her and during cross-examination tricks the pool cleaner into confessing his homosexuality, thereby undermining the case for Brooke's guilt. Callahan later makes advances toward Elle, but she rejects him. Overhearing his attempted seduction, Vivian is angered and blames Elle for using sex to further her career. Elle, frustrated by these events, contemplates leaving law school, but Professor Stromwell restores her self-esteem. Meanwhile, Brooke demands to know why Elle is missing from the team. When Emmett explains Callahan's behavior, an enraged Brooke decides to fire Callahan and Vivian learns the truth. Elle, having been hired back, conducts an aggressive cross-examination of Brooke's step-daughter Chutney, forcing her to confess that she was the one who killed Brooke's husband, but had intended to kill Brooke because she resented the fact that her father married someone her age. Brooke is exonerated and Chutney is arrested. Elle rejects Warner, claiming that she needs a man who loves her for her and not someone who acted like a bonehead. Two years later, Elle gives the graduation speech at her law school as class elected speaker. Vivian, meanwhile, has broken up with Warner, who graduated "without honors and without a job offer." Emmett has fallen out with Callahan, started his own firm, and has been dating Elle for two years and intends to propose to her.

Question: Why did Elle wear a Playboy bunny suit?
Answer: Because she thought she was going to a costume party.