Cora loves being at Mermaid Academy, and with her dolphin best friend, Sparkle! The only thing that would make it better is if she was in the same dorm as her twin sister, Isobel. Cora really wants to spend more time with Isobel but she seems more interested in her new friends. Luckily, Sparkle is always there to make Cora feel special! But then she and Sparkle spot a creature stealing jewels from the school's coral wall. Can Cora find the courage to swim out on her own?