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.

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.

When the military facility goes on high alert, Chris manages to escape, but he’s not the only one who gets out.

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.

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.

Bristol Cove mourns a loss causing Ryn to experience a new human emotion.

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.

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 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.

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.

Ben, Maddie and the other humans deal with the fallout from the attack on the oil rig, while the mermaids reacclimate 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.

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.

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.

Ben and Xander are left dealing with a shocking discovery about Chris and his connection to the military. Meanwhile, Helen and Maddie reluctantly seek 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.

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.

When another mysterious death is uncovered in Bristol Cove, Ryn suspects Tia. Maddie befriends a new acquaintance in Seattle. Helen warns the hybrids of trouble. Xander begins training and Ted revisits the past.

Ben and Maddie help Ryn guide her surrogate though a life-threatening delivery. Helen and Xander deliver a hybrid corpse to Beth at The Ranch. Xander faces danger on the high seas during a ride-along with the Bristol Cove water patrol.

Ryn returns to land to explore human motherhood. Maddie and Robb come across a gruesome discovery. Ben experiences mysterious side effects and Ted confronts Helen.

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.

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.