It’s an ordinary day, Stacy goes over to visit Barbie and tells her that she is very sad because her best friend Courtney has betrayed her Barbie decides to help her sister Stacy patch things up with her best friend, so she tells her a story about two friends named Liana and Alexa and everything they went through in the diamond castle.
The two friends went everywhere together, they only had each other. One day, while they went to the river and they found two stones with a perfect heart shape, so they decide to use them as a lucky charm and a reminder that they would be best friend forever.
The next day while they were looking for flowers in the forest, they found a poor old woman, so they decide to share the little food they had with her. The old woman was so thankful that she gave them a mirror that seemed to be ordinary, but it was everything but that!
The girls find out that inside the mirror lives a girl named Melody she was learning to be a muse, and she is inside the mirror because it’s her hiding place, she is running away from Lydia a muse that used to live with her in the diamond castle, the place where music was born and where each time you sing a new song, a new diamond would appear on the wall.
Lydia wanted to be the only muse and swore that the diamond castle would be only for her, so she transformed her flute into a magical instrument but it was black magic, and with it she spread darkness all over the castle. The other muses that lived in the castle gave Melody the key to the diamond castle so that she could escape and figure out the way to save it before being turned into stone by the evil spell.
Melody was being chased by Slyder, Lydia´s dragon, so she had no choice but to hide in a mirror she found, and ever since has been hiding until she found Liana and Alexa who sang a song that melody thought them and that was heard by Slyder who was now able to find Melody.
The dragon destroys the girl s house, but they are determined to help, so they began to search for the diamond castle, while they are thinking about what to do they meet two twins named Ian and Jeremy who fall in love with Alexa and Liana and accompany them on their journey to save the diamond castle.
During the journey they find a very skillful and terrible troll that takes the twins captive and threatens to eat them, and the only way for them to be saved is by solving a riddle and to the girls surprise once they have solved the riddle a rainbow appears! So they cross it leaving the twins behind, they continue their trip and find themselves having to overcome many obstacles, one day Liana and Alexa have a very ugly fight that puts their friendship to risk.
Liana and Alexa are still mad at each other and this makes Lydia´s magic grow stronger and puts the diamond castle as well as Melody’s life in danger; the two friends realize that they can´t be separated and that their friendship is the only thing that can beat the wickedness.
The young girls reach the diamond castle and with Melody’s help are able to open it by singing a song .when they are inside Melody is able to get out of the mirror and guides her friends to the muses’ instruments ,so that they can be played and the castle can be freed from the evil spell.
Just then Lydia tries to attack but it’s too late the music is able to break the spell and Lydia along with her dragon are turned into stone.
Once the muses are freed they thank Liana and Alexa and name them Princesses, Melody is named the castle´s muse. The two friends return to their home where they have been so happy and will always remember their good friend Melody.