As an old friend visits Helen, a new mermaid arrives leaving Ryn questioning their motives; Maddie learns the truth behind Ian's death; Ben fights for his mother's treatments; 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; Ted revisits the past.

Ryn learns the secret existence of her unborn child; Ben sets out on a dangerous path to help his mom; Xander tries to establish himself in his new career; Helen consults with Eliza to help contact Sarge.

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.

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 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.

Ryn, Ben, and Maddie must protect Bristol Cove from Tia's army; Ben's injections produce unexpected results; Xander juggles best man duties at Calvin's wedding while also helping Helen.

Tia unleashes a deadly attack on Bristol Cove; Ryn, Ben, and Maddie race against time to find a cure for Xander's mysterious illness, while anticipating Tia's next move; Baby Hope returns to land to reconnect with Ryn.

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; Helen and the hybrids help restore order.