Antonia is a secret Silver Dolphin – she's responsible for helping the creatures of the bay when their environment is threatened, by responding to the call of her magic necklace.

When her cousins come to stay, Antonia’s loyalties are divided between family and her dolphin friends. New Sea Watch members are also causing rifts and she can’t help getting swept up in rivalry with Rosie, the latest volunteer. Will Antonia end up breaking the most important promise of all?