When a quirky older woman pawns a fancy seashell necklace at her mother's antique shop on the pier, Cara doesn't think much about the story the woman spins about the wearer turning into a mermaid.

On her way home, she accidentally drops the necklace into the ocean and is swept out to sea where she meets Quay—a merman who volunteers to take her to his mother, the sea queen, to help her get her legs back.

Cara soon learns that it's Quay's eighteen birthday—a day that has been a curse for his family—and is meant to be one for her too. Now she must fight to survive the sea with Quay at her side.