Sarah pursues the missing Helena who is held in a faraway compound. Sarah must use her wiles - and acting chops - to deter a Topside investigator who threatens the Leda sisters' lives. Meanwhile, Cosima appears to be rebounding from her illness, while Alison and Donnie face newfound financial woes. But when the menace of Castor rears its head once more, Sarah and her entire family must come together to survive.
Castor pursues the Leda clones for original tissue samples. Helena, who is pregnant and imprisoned, is examined by a mysterious new villain. The Castor boys are also out to play, sexually assaulting a frightened young woman, Patty, which ultimately sets Sarah - and Art - hot on the Castor trail. Sarah gleans they may have a mysterious defect that puts their lives in peril.
Leda and Castor are both on the hunt for original clone DNA. On-the-lam, Mark and Gracie pursue new leads to find the samples that might set them free, while Art and Sarah reunite to track them down. Meanwhile, Cosima and Scott try to learn more about Castor's biology and Alison's new money-making venture is going gangbusters. A shocking revelation about the Castor-Leda connection leaves the sisters reeling, just before Sarah witnesses a deadly Prolethean-Castor face-off.
Sarah and Helena have managed to escape from the Castor compound, and arranging with Mrs. S are awaiting Benjamin's arrival in the Mexican desert to retrieve them. However, they are surprised to see that their retriever is Mrs. S herself. This meeting allows for a showdown between Helena and Mrs. S, the former who wants to kill the latter for selling her out. With Cosima's new relationship with Shay being strong, Cosima is having problems dealing with Delphine, whether it be on a professional or personal level. Cosima feels like betraying a request by Delphine without...