Barbie plays Princess Alexa , a shy, bookish royal who would rather read than rule. She discovers a magical secret door in her castle garden that leads to Zinnia, a whimsical fairy kingdom. However, Zinnia is under threat by a jealous, magical unicorn named Malucia (who steals magic). Alexa discovers she has the power to restore magic and save the kingdom, learning to be brave along the way.
For the uninitiated, Barbie and the Secret Door follows Princess Alexa (voiced by Kelly Sheridan in English). Unlike other Barbie heroines, Alexa is shy, bookish, and prefers reading in the palace library to royal duties. She discovers a magical secret door that transports her to Zinnia, a fantasy kingdom where unicorns talk and flowers dance. Here, she meets a mermaid princess named Romy and a magical creature named Nori. The plot thickens when Alexa learns she is the only one who can restore the kingdom’s magic, which has been stolen by the jealous fairy, Malucia.