Zack and Cody's school is having a musical!
When Maddie and London audition for the role of Sharpay, Maddie is the understudy because she's too 'nice'. London gets the part because the director wants Mr. Tipton to put his show 'Floss' on Broadway. In order to make London sound better, she lip syncs to Maddie's singing. When the show is going, and everybody thinks that London sounds terrific, the curtains get torn down revealing the fact that it's Maddie who is singing. They continue singing and the show is a success.