On the way to drop older brother Pierce at college, Polly and Pierce make an unexpected pitstop to the Mermaid Kingdom.

Polly learns the whimsical secrets of Sparkle Cove, a fantastical, a tropical island paradise and the source of the magical pockite responsible for her power to shrink. Soon, Polly finds that her discovery isn't as random as it seems, as she's been summoned to a mysterious location to compete for the title of "Protector," bestowed to only the fiercest of pockite wielders.