There's a new PE teacher at Mermaid School and he's very strict. Marnie Blue and her friends are horrified when he brings back Golden Glory sports day, with a super-tricky obstacle relay. It's a turtle disaster! Marnie, Pearl and Orla are put into the same team and get training. But someone has a fishy plot to win all the prizes. Can Marnie stop the cheater from ruining Golden Glory Day?