Liana and Alexa (Barbie and Teresa) are best friends who share everything, including their love of singing. While walking through the forest, the girls find a magical mirror. As they clean the mirror and sing, a musical muse named Melody appears in the mirror?s surface. Melody tells the girls about the secret of the Diamond Castle, which is the home of all music, and where three musical muses once resided until one evil muse, Lydia, tried to take the castle for herself.