The coastal town of Bristol Cove, Washington, known for its legends of once being home to mermaids and mermen for centuries, is turned upside down when a mysterious young woman appears and begins wreaking havoc upon the small fishing town to look for her captured older sister who was abducted at the hands of the local military. Marine biologists Ben and Maddie work together to find out who and what drove this primal hunter of the deep sea to land.