A few years have passed since the events of The Princess Diaries. Mia has graduated college and is ready to learn how the kingdom works. She will assist her grandmother Clarisse until it is Mia's time to transition from Princess to Queen. Unknown to Mia or Clarisse, unscrupulous Viscount Mabrey plans to take over Genovia. As a way to welcome her to the kingdom and celebrate her 21st birthday, Genovia throws a ball for Mia where she will meet several eligible bachelors. The film later reveals that there is another heir to the Genovian throne - Viscount Mabrey's nephew. Mia discovers she must marry within the next 30 days, or the Viscount's nephew will reign. Mia is shocked when she learns Mabrey's nephew is Nicholas, one of the ball's bachelors.
Mia's best friend, Lilly, flies to Genovia to help find a future husband. After non-stop searching, they settle on a Duke from another kingdom. To thwart this romance, Mabrey convinces Nicholas to woo Mia himself. Even though they argue, and Mia spends most of her time disparaging Nicholas, it is clear that they are falling for each other. Although they admit they are in love, a series of mishaps and unfortunate events masterminded by Viscount Mabrey leads Mia to believe that Nicholas has betrayed her. Because of this, she decides to go through with her wedding to the other Duke.
Nicholas learns Mabrey is behind the betrayal. He rushes to the wedding where he learns Mia couldn't make herself marry someone she doesn't love. Mia convinces the government to change the law so she is able to rule the kingdom without marrying. At her coronation ceremony, Nicholas confesses how much he loves her and the two, as they say, live happily ever after.