Shelly, Echo, Kiki, and Pearl are spending their school vacation at Neptune's castle! Shelly's aunt, Queen Edwina, has invited the girls to stay for a whole week, and while they're there the girls can't wait to visit the Royal Orca Museum, see the Palace Library, ride seahorses, and maybe even attend a royal ball.

Shelly is nervous about meeting her royal family, especially since she's never felt like a princess. But when she arrives at the castle, she is welcomed with curtsies, bows, and sea flowers. Queen Edwina even gives Shelly a sparkling tiara that she once wore as a young princess.

Just as Shelly starts to relax and have fun, the tiara disappears and the girls have to find it before the queen finds out! The mergirls have a real mermystery on their fins! But will they be able to figure out who took the tiara before Shelly's royal visit turns into a royal disaster?