Marnie Blue has settled into Lady Sealia Foam’s Mermaid School and become best friends with Pearl and Orla. And now might be her chance to shine - the Clamshell Show is coming! Marnie and Orla are both auditioning for the star role, while Pearl (who is no great singer) is sticking to playing the rock tuba. But a new mermaid wins the role, and at rehearsal she's more trouble than a tiger shark with a toothache. What's more, Mermaid Lagoon has an unwelcome visitor dropping by. Will the show go on?