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 renovated San Francisco firehouse. Her father, Philippe Renaldi (who had divorced her mother years ago but still kept in touch with Mia) 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 brother Michael Moscovitz (Robert Schwartzman).Shortly before Mia's sixteenth birthday, she learns her paternal grandmother is visiting from Genovia, a small European country. Mia meets her grandmother, Clarisse Renaldi (Julie Andrews), at the Genovian consulate for the first time in her life. Clarisse explains that the reason she wanted to see her was because of a "life-changing" problem. Mia learns that her father was the crown prince of Genovia and, due to his death, she is now the next in line and the only heir to the Genovian throne. She is shocked to learn that she is a princess and at first rejects the whole idea.Mia's mother and grandmother convince her to attend "princess lessons" in preparation for her introduction to the Genovian government at the upcoming State Dinner, under supervision from her new bodyguard, the head of Genovian security, Joseph "Joe" (Héctor Elizondo). To look more presentable to the Genovian government, Mia receives a makeover from Italian hairdresser Paolo (Larry Miller). All of this is to be kept secret until Genovia's Independence Day Ball to avoid a media frenzy.However, the San Francisco Chronicle learns that Mia is the Genovian crown princess after Paolo breaks his confidentiality agreement. Although thoroughly annoyed, Clarisse presses onward and prepares for the State Dinner. Mia attends and is publicly humiliated via her klutziness.The day after the State Dinner, Mia agrees to appear on Lilly's public-access television program Shut Up and Listen and to watch Michael's band perform at a venue that Saturday night. Mia is almost 16 so she is almost able to drive a car, (her "baby", a vintage Ford Mustang.) She takes a test drive with her grandmother and they go to an arcade. Mia asks her grandmother if her father wanted to be prince, to which she replies yes, but adds that he thought of abdicating only once when he fell in love with Mia's mother. However, he realized his love for his country was greater.While driving back to the Genovian consulate, Mia's car fails on a hill and rams into a trolley loaded with people. No one is hurt, but the police are called. To save Mia a trip to the police station (she was driving without a license), Clarisse names the police man and trolley master to the Genovian Order of the Rose (a fictional honor made up on the spot). The men are so flattered that Clarisse and Mia are given a ride back to the consulate in a police car.Josh, a cute boy on whom Mia has a crush, asks Mia to go with him to the school's annual beach party. She excitedly agrees. Michael and Lilly are both hurt when Mia blows them off for the popular kids. Mia's mother wisely points out Josh never liked her before all this princess business. Mia ignores this observation.The beach party goes well at first, but spirals out of control when the media learns of Mia's presence. Josh uses her to get his fifteen minutes of fame by publicly kissing Mia and three jealous popular girls embarrass Mia in front of the paparazzi, resulting in photos of her in only a towel.The unflattering photos wind up on tabloid covers and this displeases Clarisse. Then, Clarisse tells Mia that she could still go to the ball and invite her friends, except Josh. Joe, however, reminds Clarisse that although Mia is a princess, she is still a teenager and Clarisse's granddaughter. To reconstruct her friendships with Lilly and Michael, Mia apologizes and invites them to the Genovian Independence Day Ball, where she must reveal whether or not she wants to accept her duties as a princess.Upon learning she must personally and publicly renounce her throne, the terrified Mia plans to run away. She is stopped when she finds a sixteenth birthday gift (the titular diary) and a letter from her father written before his death. Touched by his words, she changes her mind and makes her way to the ball. Her car breaks down on the way, but she is saved by Joe, who arrives with a limo (having suspected her runaway plans).When they arrive, Mia makes a speech announcing her acceptance of the Genovian throne. She shares her first dance with Michael, who then takes her outside to the palace garden where they kiss. Clarisse and Joe are seen holding hands, signifying the start of a relationship. The last scene shows Mia on the plane with Joe and her cat. Mia is writing in her diary, explaining that she is moving to Genovia, her mother is moving with her, and Lily and Michael will visit her during the summer. When Mia looks out the plane's window she sees the beautiful palace of Genovia below.
Output: Question: How old in Mia Thermopolis?, Answer: Fifteen-year-old