More mermaids appear in the coastal town of Bristol Cove … but are they seeking revenge or is something more catastrophic driving them out of the sea? Marine researchers Ben and Maddie continue to establish a bond with mermaid Ryn, not only to protect her, but to try to help save her entire civilization.
With more mermaids landing at Bristol Cove’s shores, the threat of discovery increases. After his encounter with the siren song, Ben tries to cope with the after effects. Maddie’s mom returns to town and tries to connect with her daughter. Meanwhile, Ryn attempts to adapt to life on land.
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.
Ryn fights to maintain leadership of the pack when her loyalties are questioned over her friendship with Ben and Maddie. Ben interferes in a mermaid ritual and Helen discovers more about her family lineage.
Ryn decides they need to find another solution for housing mermaids as the pack dynamics remain precarious. Ben becomes a mentor to Levi as he tries to learn how men behave on land.
Ryn continues to try to teach the other mermaids to be more human in order to better assimilate, but some are starting to resist her, causing Ryn to wonder if she herself is becoming too human. Maddie finds herself in a dangerous situation trying to help her mom. Meanwhile, Ben and Xander reconcile in order to work together to get the mermaid pack back into the water.
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.
Tensions are rising following a shocking discovery. And living in a tank rather than the ocean is starting to have an adverse effect on many of the mermaids. Hoping to save her kind, and help her human circle, Ryn makes a deal with a complicated ally.
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.