Emily Windsnap has always lived on a boat but her mother has never let her learn to swim. During her first swimming lesson, Emily discovers a shocking secret—when she gets in the water, her legs transform into a shimmering mermaid tail! Her underwater adventures lead her to meet a mermaid named Shona and uncover a family mystery involving her missing father. Emily must navigate between two worlds to reunite her family and protect the secret mermaid kingdom from a threatening force.