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 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.

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.

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.

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.

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.

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.