Amelia (Mia) Mignonette Grimaldi Thermopolis Renaldo has just learned that she is heir to the throne in Genovia when she is also hit with the news that she is about to be a big sister in Meg Cabot's novel "Princess in the Spotlight". In this story, the self-conscious teenager learns that not everything is as it appears to be. During the course of the two-week period that Mia's journal covers, she becomes better acquainted with both the good and bad qualities of being a public figure. She also makes surprising discoveries about both her best friend and her father that improve her opinion of people in general.

Mia's journal begins as she is preparing one morning for school and learns that her unwed mother is pregnant. Mia spends the rest of the day wondering how her mother and algebra teacher could have been so careless, and thinking about things she needs to encourage her mother to do, and not do, now that she is pregnant. Meanwhile, Mia is faced with a television interview with Beverly Bellerieve, of the television show Twenty-Four/Seven. In this interview, Mia accidentally tells that her mother is expecting. She also talks openly about her high school, causing issues for more than one person.

Mia's paternal grandmother, who is in New York giving Mia princess lessons, complicates matters for Mia by deciding to plan a wedding for Mia's mother, who only wants a small ceremony at city hall. Mia tries to convince her grandmother that her mother isn't interested in the large, celebrity-filled ceremony that she is planning, but her grandmother will not listen. Mia turns to her father to stop the wedding, but is worried that he is too afraid of his mother to go against her wishes.

At this same time, Mia learns that she has a secret admirer. Mia hopes the admirer is Michael Moscovitz, her best friend's brother whom she has had a crush on for years. The novel comes to a climax as Mia's mother and algebra teacher disappear the day of the wedding. It turns out that Mia's father helped them plan an escape and elopement in Cancun, a trip that the three of them kept secret to keep Mia out of trouble with her grandmother.

When Mia's grandmother flees to the privacy of her hotel suite to hide because Mia's mother does not show up at the wedding, it is Mia that convinces her to come back to the "wedding" reception. She uses on her grandmother the same lessons that the older princess taught her granddaughter during princess lessons. She reminds her grandmother that a princess has to be strong and must not hide behind her wealth and privilege.

During this same time period, Mia's English teacher has given her class an assignment of keeping a journal for class. One of the assignments is to describe a profound moment. Although Mia does not ever recognize the profound moments in her life, she has several in the course of these two weeks. She keeps looking for something a teenager would consider profound, like seeing Michael with his shirt off.