Marnie Blue and her friends are horrified to discover they have a new P.E. teacher at Mermaid School, and he's very keen to improve everyone's fitness. Worse, he's bringing back the Golden Glory sports day. It's a turtle disaster! Marnie, Pearl and Orla are put into the same team to compete in ultra fishball, the obstacle relay and the seahorse riding events. But someone has a fishy plot to win all the prizes. Can Marnie stop the cheater from ruining Golden Glory Day?