It is summer and Clumsy Princess Grace is back at Tall Towers Academy for her third term. As the sun shines bright on Coronet Island, Grace is desperate to leap into the beautiful shell-shaped pool for swimming lessons - with real mermaids! But her best friend Scarlet is not so sure. She is terrified of the water and begins to act very strangely. As Grace follows her to the beach one moonlit night, an enchanted mermaid mystery begins to unravel. With the help of dolphins, jewels and secrets from long ago, can Grace protect her friend and save a mermaid too?