Erica Shepherd is a brilliant former CIA operative, now known as the most notorious traitor in American history serving life in a Supermax prison. Against every fiber of his being but with nowhere else to turn, FBI Agent Will Keaton enlists Shepherd to help track down a fiercely dangerous and elusive criminal she knows all too well. While Shepherd and Keaton have different motivations for bringing the enemy to justice, they both know that to catch a spy… they must think like one.

The coastal town of Bristol Cove is known for its legend of once being home to mermaids. When the arrival of a mysterious girl proves this folklore all too true, the battle between man and sea takes a very vicious turn as these predatory beings return to reclaim their right to the ocean.