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.