A missing magical pony, lost treasure, a mysterious sea monster and a vanished princess. When there's a mystery to be solved, the young mermaid detectives are always right on the case! It's autumn, and the friends have volunteered to help the water fairies out with some chores at the underwater gardens. While they work, one of the fairies tells them the sad tale of the beautiful mermaid Aroona and her human love, Sir Topaz. All the mermaids believe the tale - all except Jasmine, who thinks it's just a silly story. But then she discovers the trail that leads to Aroona's lost treasure, and she sets off to discover the truth.