The crook and the crown

Shelly Siren and her friends are excited to spend their school vacation at King Neptune's royal castle, where they plan to explore the museum, library, and maybe even attend a royal ball. Shelly, nervous about her royal family, receives a sparkling tiara from Queen Edwina. However, when the tiara mysteriously disappears, the mergirls must solve the mermystery before Shelly's royal visit turns into a disaster.