When there's a magical mystery to be solved, the young mermaid detectives are right on the case. The friends volunteer to help the water fairies tend to their underwater gardens. While they work, a fairy tells the sad tale of the beautiful mermaid Aroona and her human love, Sir Topaz. All the mermaids believe the tale--except for Jasmine, who thinks it's just a silly story. But when Jasmine discovers the trail that leads to Aroona's lost treasure, she sets off to discover the truth.