Not one of the merkids in Shelly Siren's third-grade class can believe the shell-shattering news: Shelly is a princess! A real princess! It's been a well-kept secret in Trident City, but now the word is out, and Shelly doesn't know how to act! Should she start wearing a glittery crown? Or move to a grand undersea palace? Will her friends have to call her Your Highness? Being a princess may be exciting, but Shelly's not sure she can truly play this royal part.