Snooty Deidre Shannon writes in her blog "Dishing with Deidre" that since eight is half of sixteen, she's entitled to a Super Sweet Half-Sixteen party this year! And since the Shannons have a pool, she's decided to make mermaids her theme. Nancy, Bess, and George are invited to the big bash, where all party guests must dress in under-the-sea-themed costumes. For her grand entrance, Deidre dresses as a mermaid and is carried out on a giant half-shell by six teenage "sea creatures." The garden boasts hedges clipped into the shapes of various types of ocean life. Deidre's father even booked "Queen Mermaid Marissa" to perform in their pool, fin and all. But when a squirmy snake is found swimming in the pool too, Queen Marissa leaves in a huff! Who would sabotage the mermaid pool performance? Nancy, Bess, and George intend to find out!