- Directors: Garry Marshall
- Producers: Debra Martin Chase, Whitney Houston, Mario Iscovich
- Writers: Meg Cabot, Gina Wendkos
- Genres: Comedy, Family, Romance
- Actors: Anne Hathaway, Heather Matarazzo, Julie Andrews, Mandy Moore
Mia Thermopolis (Anne Hathaway) is a fifteen-year-old tenth grade private school student who lives with her mother Helen Thermopolis (Caroline Goodall) and her cat, Fat Louie, in a San Francisco firehouse. Her father, Philippe Renaldi (who was divorced but in touch with her) died two months earlier in a car crash. Although Mia is an average student, she is very unpopular, but has two good friends: Lilly Moscovitz (Heather Matarazzo) and Lilly’s older brother, Michael Moscovitz (Robert Schwartzman), who has a crush on Mia. Mia is regularly teased by her peers for her gawkiness and frizzy hair, mostly by Lana Thomas (Mandy Moore), a fellow tenth grader who is captain of the school cheerleading team, and her friends. Mia is jealous of Lana because she is in a relationship with Josh Bryant (Erik von Detten), a twelfth grader “Backstreet Boy clone” (as her mother describes him) on whom she has had a crush since grade school.
Shortly before Mia’s sixteenth birthday, she learns from her mother that her paternal grandmother is visiting from Genovia, a fictional country in Europe (which bears some similarity to the actual Republic of Genoa, had it not become part of unified Italy). At first Mia does not want to see her, but agrees to after some persuasion. The next day, Mia meets her grandmother, Clarisse Renaldi (Julie Andrews), at the Genovian consulate for the first time, who explains that the reason she wanted to see her was because of a “life-changing” problem. She learns that her father was the crown prince of Genovia and she is the next crown princess of Genovia and the only heir to the Genovian throne because of her father’s death. She was shocked to learn that she is a princess and exclaims “shut up!” to her grandmother’s disapproval. Mia quickly leaves the house and returns home, after which she begins fighting with her mother for not telling her when she was younger.
Josh asks Mia to go with him to the Baker Beach Bash, the school’s annual beach party, and she excitedly agrees. She remembers to tell Michael that she will watch his band the following Saturday night, but forgets to tell Lilly of the change. Lilly was very angry when she knew about it. The beach party goes well at first, but spirals out of control when the media learn of Mia’s presence. Josh publicly embarrasses her by kissing her in front of photographers, and Lana helps them take pictures of her clad only in a towel. This causes a media frenzy and displeases Clarisse. Clarisse then gets a wake up call from Joe, telling her that she has to remember that although Mia is a princess, and the heir to her throne, Mia is still her granddaughter. To reconstruct her friendships with Lilly and Michael, Mia invites them to the Genovian Independence Day Ball, where she must reveal whether or not she wants to accept her duties as a princess. Originally intent on running away to Colorado because she wants to avoid renouncing her throne with a speech (Mia is terrible at public speaking), Mia finds a sixteenth birthday gift and letter from her father, written before his death. After reading the letter, she changes her mind and makes her way to the ball, as it begins raining. Her car’s roof will not go up though, so she is stuck driving in the rain and her car breaks down when she attempts to go up an incredibly steep hill. Joe finds her drenched halfway to the event. Upon arrival, Mia makes a speech announcing her acceptance to the Genovian throne. She is happy to see that her friends attended, and kisses Michael in the garden. Clarisse and Joe also walk out of the Genovian consulate holding hands, showing sparks of a relationship. The film ends with the cast dancing informally, and Mia on an airplane arriving in Genovia as she narrates the coming summer’s events.