Tia unleashes a deadly attack on Bristol Cove. Ryn, Ben and Maddie race against time to find a cure for Xander's mysterious illness, while anticipating Tia's next move. Baby Hope returns to land to reconnect with Ryn.
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.