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.
Ryn seeks help only to learn the secret existence of her unborn child. Ben sets out on a dangerous path to help his mom, while 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 inactive person 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 unhappy 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. Xander struggles to make amends with Annie.
Ryn, Ben and Maddie must protect Bristol Cove from Tia's army. 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.
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. Ted struggles to accept Ben's reality.