Lizzie McGuire (Hilary Duff), the blonde haired klutz/dork/loser, among other not-so-nice-things, has just graduated middle school with her best friends David 'Gordo' Gordon (Adam Lamberg), and Miranda Sanchez (Lalaine). During the event, she makes a shambles of the stage with her brother, Matt (Jake Thomas), all the while recording it to add to his collection of his sister's blunders. As a post graduation/pre high school surprise, the newly graduates, along with other friends and rivals, get to go to the beautiful Rome, Italy with the not so beautiful high school principal, Mrs.Ungermeyer. Miranda goes on a family vacation, so it's just the two friends. That's when a tiny romance begins to blossom.
When they get there, the kids expect fun, romance, and in some people's cases, tons of spaghetti. What they get is a droning tour with Ungermeyer. When Lizzie meets a famous Italian pop star, Paolo, she gets a bit of fun and adventure mixed with drama and lying, when she finds out she must replace Isabella, the brunette look alike pop singer that duets with Paolo, at the Italian Music Awards. After Kate (Ashlie Brillault}, Lizzie's rival, finds this all out and swears to keep things a secret from Ungermeyer, Gordo finds out Paolo is using Lizzie so he won't get sued.
She won't believe him, yet he continues to cover for her, who knew about this since the beginning. Things get a little to out of control, and soon Ungermeyer comes to the obvious that Lizzie has been faking sick to sneak out. Gordo tells Ungermeyer that it was him sneaking around, which gets his a cozy seat on the next plane to America. But as he's getting on the plane, Isabella, who just returned from an island, is getting off a plane.
She found out about Lizzie and demands her bodyguards tell her who she is. But the only one who can tell her is Gordo. Meanwhile, Matt is at home and, with the help of his friend Melina Bianco (Carly Schroeder), tries to figure out a way on how to cash in on Lizzie's new-found success. Pretending to his parents that he misses his older sister, he convinces them so they can go to Rome and get her, and he can make money of the embarrassing tapes. On the plane, their parents figure out what Lizzie's trip held, and Matt must explain all he knows in a 14 hour trip. The night that Lizzie and Paolo perform, Isabella and Gordo find Lizzie getting ready backstage.They tell her everything, and now she believes them. Lizzie tells Isabella to go ahead, but Lizzie must still perform after the brunette singer states that she is not blonde, and Paolo will find an excuse to leave. So as they go on, Isabella comes up with a clever plan and tells the operators to turn up Paolo's microphone in the middle of the song, so the audience can hear his horrible voice instead of his gorgeous lipsynching. When he's caught, he runs off and tells paparazzi it was just his voice. But no one believes him, so thats the end of Paolo.
Isabella introduces Lizzie as her new American friend. They start an upbeat pop duet of the original Paolo Lizzie ballad, just as Ungermeyer, the students, and Lizzie's family come in to watch her. Then Isabella leaves Lizzie to sing by herself. She tries to run off but Gordo tells her she can do it. She pretty much sums up her trip to Roma in song. The movie ends with Lizzie getting grounded, Matt being wanted out of country for blackmailing family, Kate and Ethan (Clayton Snyder), the heartthrob, enjoying spaghetti, and a sweet kiss on the balcony of the hotel shared between Gordo and Lizzie