Dale copes with the fallout from Maddie and Ryn's encounter with the man from Susan's past, while Katrina continues to persuade the rest of the mermaid pack to get on her side - even if that means brute force against both mermaids and humans.

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.