In this fun, underwater adventure, Barbie and her sisters visit Ken at his summer internship where he works at a coral reef researching dolphins. While diving and exploring the reef, the sisters discover rare, rainbow-colored dolphins who only visit once a year. This amazing find encourages them to learn more about preserving the coral and all the sea life that lives there. That's when they meet Isla, a mysterious new friend who's also on a mission to save the coral. What seems like a simple task turns out to be quite the adventure when it's revealed Isla is really a mermaid. But there's no time to celebrate having a mermaid for a friend, because Isla needs the help of Barbie and her sisters, and the gemstone dolphins to save the day.