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. But the mergirls end up with a real mermystery on their fins when the tiara disappears!