Plot
While Barbie and Teresa are playing music, Barbie’s sister Stacie storms in, having had a fight with her friend Courtney. Barbie and Teresa tell Stacie a story to teach her the power of friendship.
Liana and Alexa are two friends who share a love for music, and live in a cottage where they grow and sell flowers for a living, but have very little for them to eat. One day, they find two heart-shaped stones and make them into necklaces to symbolize their friendship. That evening, a storm occurs, destroying their garden. The next day, Liana offers her food to a poor, old woman who gives them a mirror in exchange. Later, the two hear a voice from the mirror and find a girl inside it named Melody. The trio bond over music and Melody teaches them a song; but they are soon attacked by a dragon named Slyder. They manage to escape him, but their cottage catches fire and burns down.
Melody explains that she was once the apprentice of the three Muses, Dori, Phaedra, and Lydia, who lived in the Diamond Castle. Lydia turned evil and planned to take the Diamond castle for herself. After hiding the castle and entrusting Melody with its key, Lydia turned the other Muses into stone; Melody, fleeing Slyder, hid and inadvertently trapped herself inside the mirror. Melody tells Liana and Alexa that Lydia can be defeated if they play the Muses' instruments. Liana insists on helping Melody and Alexa reluctantly agrees. On their journey, Liana and Alexa adopt two puppies, naming them Sparkles and Lily. The girls are confronted by Lydia who orders them to hand over Melody. When they refuse, Lydia attempts to hypnotize them with her flute but they are shielded by the stones on their necklaces (revealed to be from the Diamond Castle). The girls escape with the help of twins Ian and Jeremy who join them on their journey; but are separated when Liana and Alexa are carried off by a magic bridge.
Liana and Alexa arrive at a mansion and are greeted by its hosts who, hypnotized by Lydia, tell them that they are the mansion's prophesied owners. Alexa suggests abandoning their quest and remaining in the mansion, causing her and Liana to argue. Liana and Melody leave and Alexa angrily takes off her necklace, whereupon she is captured and hypnotized by Lydia. Liana and Melody are abducted by Slyder. Alexa is ordered to leap off a ledge if Melody does not surrender the castle's key. Melody agrees, to save her and Liana; but as she and Lydia leave, Slyder knocks them off the ledge anyway. Liana manages to hang on to the ledge and Alexa, before returning Alexa's necklace and breaking Lydia's spell.
After reconciling, Liana and Alexa reunite with the twins and go after Lydia. While the twins distract Slyder, Lydia creates a whirlpool and orders the girls to drown themselves; they feign being hypnotized, enabling Liana to grab Lydia’s flute. Lydia threatens to break the mirror, but Melody shatters it from within to stop her. The mirror is thrown into the whirlpool but is saved by Sparkles and Lily. When Lydia's flute drops in the water, she wades in to get it; but the waterlogged flute backfires and Lydia seemingly disappears.
Thinking the Diamond Castle can bring Melody back, the girls realize that the song Melody taught them is the key. They sing together and the Diamond Castle reappears. Stepping inside, Liana and Alexa are magically given new dresses and Melody is freed from the mirror. The girls enter the music room, but Lydia returns with Slyder. The three of them play the Muses' instruments and sing together, overpowering Lydia and turning her and Slyder to stone. The other Muses are freed and Lydia's spells are all undone.
Liana and Alexa are crowned princesses of music and Melody becomes a Muse. Melody invites them to live in the castle, but they choose instead to return to their old life with their new pets; the Muses give the two magic seeds which blossom instantly into golden flowers. Back to the present, Stacie realizes nothing can break true friendship and goes to apologize to Courtney; Barbie and Teresa happily resume playing music.
