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.

Ryn discovers a refuge of injured mermaids that escaped Tia’s violent takeover. Ben and Xander get caught in a battle with Tia’s troops. Maddie feeds Robb half-truths to secure his discretion. Ted explores his family lineage.

In an effort to save Hope, Ryn and Ben confront Tia in an undersea battle between the mermaid tribes. Maddie and Robb work to find a cure for Xander, while Helen and the hybrids help restore order. Ted struggles to accept Ben’s reality. Season finale.

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.

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

When Ryn is taken in for questioning, Maddie rushes to her rescue. Meanwhile, Ben and Xander return to the deep sea to search for answers, only to be met by an uninvited guest.

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.

Ryn, Ben, Maddie must recruit new allies in the fight against Tia. Helen seeks answers when she’s haunted by Donna’s spirit. Tia obtains dangerous weapons she plans to use in her fight. Xander struggles to make amends with Annie.

Ryn decides they need to find another solution for housing the mermaids, as hiding them becomes increasingly difficult. Ben becomes a mentor to Levi as he tries to learn how men behave on land, while Maddie continues to struggle with her mother’s renewed presence in her life.

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.

Donna comes ashore with two other members of her colony to bring Ryn back home. When mermaids once again begin wreaking havoc throughout the town, Dale tries to put an end to it once and for all.

As an old friend visits Helen, a new mermaid arrives leaving Ryn questioning their motives. Maddie learns the truth behind Ian’s death, as Ben fights for his mother’s treatments. While Xander is inspired to take on more responsibility.

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

Dale and the local police’s investigation starts pointing directly to Ryn, meanwhile Xander and Calvin’s search for Chris heats up when they locate the nurse who works at the military bunker.

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.

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.

Ben begins to investigate some suspicious overfishing, meanwhile Decker ramps up his efforts to find his escaped mermaid and shows the true extent of his influence.

Mating season turned out to be less successful than everyone had hoped, leading them to 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.

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.

When Ben and Maddie attend his mother’s fundraising gala, an unexpected guest shows up and offers to put an end to the overfishing that’s threatening the lives of the mermaids in Bristol Cove.

Ryn, Ben and Maddie must protect Bristol Cove from Tia’s army. Meanwhile, Ben’s injections are producing unexpected results, which alarms both him and the others. Xander juggles best man duties at Calvin’s wedding while also helping Helen.

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 the sea together to find the cure. Meanwhile, Helen and Ted connect over their shared family histories.