The movie begins with Barbie and Teresa writing a song together on their guitars when Barbie's sister Stacie storms in, angry at her friend Courtney. Barbie decides to tell Stacie the story of two friends named Liana and Alexa, who also get angry at each other.
The two best friends live in a hut beside a beautiful garden and plant flowers to make a living. They also share a love of music. One day, they find two heart-shaped stones which they make into necklaces to symbolize their friendship, not realizing that they are trinkets from the mythical Diamond Castle. The next day, an old woman gives them a mirror after they share the last of their food with her. The mirror is the hiding place of Melody, an apprentice muse. Melody is on the run from Lydia, one of three muses who lived in the Diamond Castle. Lydia became evil and turned the other two muses to stone, but not before they hid the Castle and left the key with Melody. If Lydia finds the Diamond Castle, all music will end, and with it eventually the world. Liana and Alexa coax a song out of Melody by singing, and Lydia's minion,
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They do, which frees the boys and reveals a rainbow bridge that carries the girls off, leaving the boys behind. Liana and Alexa soon run out of food. They find a mansion and go to ask for shelter, but the servants there (bewitched by Lydia) tell them that they are the long-awaited mistresses of the house. Alexa wants to stay, but Liana wants to honor the promise they made to Melody, and they argue. Liana leaves and Alexa stays, ripping off her necklace in anger. Slyder arrives and takes Alexa to Lydia. After determining that Melody is not with Alexa, Lydia uses her flute's powers to find Liana, and because Alexa is not wearing her necklace, she falls under Lydia's spell. Alexa tells Lydia that Liana and Melody are heading to the seven stones, where the Diamond Castle is