High school sophomore Princess Mia records in her diary her struggles with geometry, the expectations of being a college student's girlfriend, running for president of the student council, and a potential ecological disaster in her native land, Genovia.

Princess Mia, struggling to come to terms with her recent breakup with Michael and prepare for an important speaking engagement at a gala for Domina Rei, finds solace and guidance in the diary of a teenage princess for Genovia, and discovers a secret that may change her country's history forever.

Princess Mia's junior year in high school begins with a difficult course load, a crazy student council race, Grandmere's search for temporary lodging, and the news that her boyfriend is moving to Japan for a year.