Siren

A new environmental threat wreaks havoc on Bristol Cove's shores, bringing more mermaids to land and more threat of discovery. Ben continues to struggle with the effects of the siren song. Maddie's mom returns to town and Ryn attempts to adapt to life on land.

Ben and Maddie work with Ryn to try and keep the pack hidden from the rest of town, but the new mermaid's quick degradation on land forces a ticking clock as they figure out how to get them back to sea.

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. Maddie comes up with a solution to study the siren song and its affects.

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 and Maddie continues to struggle with her mother's renewed presence in her life. Xander utilizes some new leverage to try and locate Levi.

Ryn continues to try to teach the other mermaids to be more human on land in order to better integrate, but some are starting to resist her. Maddie finds herself in a dangerous situation trying to help her mom, who has fallen off the wagon. Ben and Xander work to reconcile in order to work together and get the mermaid pack back into the water.

Katrina continues to try and get the rest of the mermaid pack on her side. Ben, Maddie, and Helen try to help Ryn maintain control and try to come up with a strategy to stop the oil company’s offshore plans.

Tensions are high following the discovery of Sean McClure's body. Elsewhere some of the mermaids are starting to become sick as the tank is not as effective as the ocean. Ryn makes a deal with a complicated ally in order to help both her people and humans.

Humans and mermaids come together to enact an eco-terrorism attack plan to stop the oil company, but their efforts are not without losses. Cami enacts revenge on her mother's murderer. Facing an uncertain future Ben, Maddie and Ryn come together in a romantic farewell.

Ben, Maddie and the other humans deal with the fallout from the attack on the oil rig, while the mermaids re-acclimate to their ocean home. Ryn comes back to land to follow through on an agreement she made with the military, despite not knowing what their ultimate goals are.

With Elaine Pownall's health worsening, she seeks treatment from the military's medical trial using mermaid cells, but the procedure doesn't come without side effects to the mermaids. Meanwhile, Helen and Rick learn more about the hybrid status, their history in Bristol Cove, and discover they are far less alone than Helen once believed.

Maddie and Ben contend with how to help Ryn knowing that in order to help Elaine get better, they will need to continue the procedures on Ryn. Meanwhile, when one of the group gets sirened, there is more drive than ever to get to the bottom of how the siren song really works – further piquing the interest of the military of how Ryn can be of use to them.

Ryn goes back to the ocean to get help for Ben and Maddie's conditions, but discovers that continuing to go back may get harder than she realized the longer she's on land. But when they learn that there may be a special place in Ryn's homeland that could help them, Ben, Maddie and Ryn must all venture under theseatogether to find the cure. Meanwhile, Helen and Ted connect over their shared family histories.

Mermaid mating season has begun, with Ryn knowing she will need to go back to the sea to help her colony. The hybrids convince one of the mermaids to come with them to a remote location, leading Helen and Ben on a hunt to find them. A journalist turns to Xander for help putting the pieces together of the night of the oil rig disaster, but Xander is torn on where his loyalties lie and what helping him could mean for his future.

Mating season turned out to be less successful than everyone had hoped, leadingthemto question the possibility of the mermaids procreating on land. Ryn brings a new mate ashore for the good of the colony, but it raises emotional questions for Ben and Maddie.

Ben and Xander are left dealing with a shocking discovery about Chris and his connection to the military. Meanwhile, Helen and Maddie reluctantlyseek help from the hybrids as the threat of extinction looms over the mermaids.Ryn encounters the journalist who continues to chase his theories on mermaids to increasingly desperate ends.

Ben faces a choice that has rippling consequences amongst mermaid, hybrid and human alike, as the truth about mermaids is exposed.