The coastal town of Bristol Cove, once known for being home to mermaids, is turned upside down with the arrival of a mysterious girl.

Ben and crew of The North Star uncover a device emitting a strange sound. Meanwhile, Ryn finds shelter in the home of the local folklore expert.

In the season finale, the enchanting siren song takes hold of Ben and Decker. Meanwhile, Dale must answer for the escalating crime in Bristol Cove.

Ben, Maddie and Ryn desperately try and keep the newly arrived mermaid pack hidden from the rest of the town, but the new mermaids’ quick degradation on land forces them to get them back to the sea unnoticed during the town’s Mermaid Beauty Contest. Elaine grows more suspicious of Helen and her connections to their family.

Mermaids and humans join forces in a radical plan to stop the oil company, but their efforts have a devastating effect. And facing an uncertain future, Ben, Maddie and Ryn come together for a romantic farewell.