Princess Rosalinda Maria Montoya Fiore (Demi Lovato) is about to be crowned queen of the small nation of Costa Luna. General Kane (Johnny Ray Rodriguez), the dictator of neighbouring country Costa Estrella, infiltrates her palace with his agents during her coronation rehearsal, and attempts to capture the royal family and take over the country. Joe Mason (Tom Verica), an agent of the Princess Protection Program, a secret organization funded by royal families that looks after endangered princesses, whisks her away to safety via helicopter. Kane’s agents however succeed in capturing her mother, Queen Sophia.
The Princess Protection Program hides Rosalinda in Mr. Mason’s home in Louisiana, where she is to pretend to be a typical American teenager named Rosie Gonzalez. She meets Mr. Mason's daughter, Carter Mason (Selena Gomez), an insecure tomboy who works at the family bait shop and dreams of going to the homecoming dance with her crush, Donny (Robert Adamson). Though Carter initially treats Rosie with annoyance and barely-masked hostility, she warms up to her after Rosie explains her situation, and the two become close friends. Carter teaches Rosalinda to act like a normal girl and Rosie shows Carter how to disarm those that scorn them by behaving as a princess. Rosie soon becomes popular at their high school.
In an attempt to trick Rosalinda into exposing her location, General Kane announces plans to forcibly marry Rosalinda’s mother. Rosalinda is distraught and tells Carter that she has decided to secretly return home. Knowing Costa Luna is still too dangerous, Carter secretly devises a plan to pose as Rosalinda and then use herself as bait to lure Kane into capture. Mr. Elegante, Rosalinda's royal dress maker, tells Kane that Rosalinda will be attending the homecoming dance and will be wearing a blue dress that he actually sends to Carter. In the meantime, Rosalinda agrees to help Carter behave like a princess by helping a group of girls dress up for the dance; The girls all wear masks, which helps Carter disguise herself as Rosalinda.
According to plan, Kane and his agents mistake Carter for Rosie and lead her to Kane's helicopter the night of the dance. However, Rosalinda narrowly discovers and ruins the plan by exposing herself to Kane, insisting that this is not Carter's fight. Fortunately, agents of the Princess Protection Program, including Mr. Mason, have been waiting inside the helicopter and rescue both girls. The PPP agents quickly apprehend Kane and his henchmen and turn them over to the international authorities.
At the end, Carter realizes what "a jerk" Donny is and goes to the dance with Ed, her best friend who has had a crush on her for a long time. Rosie is crowned Queen of Costa Luna with Carter, Mr. Mason, Ed, Rosalinda's mother, and Mr. Elegante in attendance while the song "Two Worlds Collide" plays.
- Demi Lovato as Princess Rosalinda Maria Montoya Fioré/Rosie Gonzalez, a young princess who is heir to Costa Luna's throne. When she arrives to Louisiana, she has trouble fitting in. At the end of the film, she is crowned queen.
- Selena Gomez as Carter Mason, a tomboy and daughter of Major Joe Mason who has a crush on Donny. She teaches Rosie how to be a normal girl from Iowa.
- Carrie A. McSoda as Milly
- Tom Verica as Major Joe Mason, works for the Princess Protection Program.
- Sully Diaz as Queen Sophia Fioré, the mother of Rosalinda and Queen of Costa Luna.
- Johnny Ray Rodriguez as General Magnus Kane. The General and king from Costa Estella who is obsessed with gain control of Costa Luna. He even goes as far as trying to kidnap Rosalinda.
- Jamie Chung as Chelsea Barnes who is obsessed with being homecoming queen, even going as far as threatening to expose Rosie, jumping into a pool, texting people to not vote for Rosie, humiliate her, and more horrible things.
- Nicholas Braun as Edwin aka Ed, who is Carter's dorky friend. He has a crush on Carter.
- Robert Adamson as Donny, Carter's temporary crush since the third grade. Carter stops liking him after she realizes what a huge jerk he is. Before prom, he couldn't even remember her name and she repeatedly reminded him. He has a small crush on Rosie, but only for her beauty.
- Samantha Droke as Brooke Angels is Chelsea's dim-witted sidekick.
- Kevin G. Schmidt as Bull is the friend of Donny, Chelsea, and Brooke.
- Talia Rothenberg as Margaret
- Molly Hagan as The Director
- Dale Dickey as Helen Digenerstet
- Ricardo Alvarez as Mr. Elegante, Costa Luna's royal designer who helps arrest General Kane.
- Brian Tester as Principal Burkle. He is the principal of the school Carter goes to.
The film introduced two new songs: a duet recorded by Gomez and Lovato called "One And The Same" and a song recorded by Mitchel Musso called "The Girl Can't Help It." Both songs are featured on the Disney compilation album, Disney Channel Playlist, which was released on June 9, 2009. The film also includes Lovato's song "Two Worlds Collide", which was first featured on her debut album Don't Forget. Disney Channel promoted the movie's premiere weekend by offering never-before-seen episodes of their original series' Wizards of Waverly Place and Sonny With a Chance as an online reward if viewers can correctly count the number of times the words "princess," "princesses," and "princesa" are spoken during the movie and enter the correct number, 86, into a section on their website.
The DVD was released on June 30, 2009 in the US. It also features the music video for Gomez & Lovato's duet "One and the Same" and a behind-the-scenes look at the movie. The DVD was released on June 18, 2009 in Germany and four days later in the UK.
Laura Fries of Variety magazine describes the film as being "light as a summer breeze on the Louisiana bayou". Although the film does not stray far from the Disney formula, she praises writer Annie DeYoung for providing young girls a nice message about self-esteem. She says the film should be a hit with the channel's target audience.
The premiere was watched by 8.5 million viewers, making Disney Channel TV the most-watched network in the time slot, with nearly double the viewers of CBS at that time.