It is Marnie Blue's very first day at Lady Sealia Foam’s Mermaid School, and she's terrified. What if she can't make any new friends, or if she's asked to ride a seahorse? What's more, her famous diva aunt Christabel was a notorious rebel at the same school, and got up to all sorts of naughtiness. Marnie is soon branded a troublemaker by her grumpy octopus music teacher and wants nothing more than to clam up. But when a fellow pupil finds herself in danger, it's up to Marnie to show that daring runs in the family and swim to the rescue.