A wounded Sarah fights to stay alive on the island, where Cosima wakes up in a community of people who follow the Neolution founder P.T. Westmoreland -- who they say is still living at 170 years of age. On the mainland Art is conscripted into aiding in the hunt for Alison, Donnie, and Helena, who are hiding out in the woods.

Sarah is reunited with Kira only to be given a heartbreaking ultimatum by Rachel: return home to a normal family life but allow Kira to be studied by the Dyad Institute. While Felix goes in search of M.K. for help Helena decides she cannot trust her doctors, and on the island Cosima comes face-to-face with the legendary founder of Neolution.

As the fall fair arrives Alison struggles to come to terms with the life she has lost at Bailey Downs, including the death of her friend by her own negligence -- while Detective Enger and the Neolutionists, searching for Helena, close in on the secrets hidden at the Hendrix home.

As Sarah grows more frustrated with Kira's involvement with Rachel, Mrs. S takes Sarah on a mission to pose as a psychologist in order to locate a Neolution defector. On the island, Cosima tries to uncover the secret of the danger living in the woods.

As Cosima draws closer to the secrets of Westmoreland's Neolution work, the island villagers organize a search party to find what's living in the woods. Sarah tries a new strategy for bonding with Kira, and Delphine returns to the island in time for an unsettling family dinner.

After Krystal brings information about a cosmetics company, Sarah plans to impersonate her to learn more about its connection to Dyad. Susan and Ira are caught off guard when Virginia Coady arrives on the island to accelerate P.T.'s agenda.

As Westmoreland brings Rachel into the Neolution fold she grows worried about whether her life is truly her own. Coady brings someone new to the island, and on the mainland the team searches to uncover their enemy's true identity.

With Neolution and Dyad in disarray following Rachel's actions Mrs. S orders everyone to take a breather, as the family comes together to support Felix's art opening. And after she obtains enough information to destroy Dyad and its board members, Rachel plans for her new life with an old ally. Elsewhere, Helena is suspicious of her newest visitor.

After a devastating loss Felix and Art go in search of a lead on Helena, who has been abducted by Neolution as her babies come to term. Westmoreland orders Coady to put an end to the Castor line once and for all.

With Helena on the verge of delivering her babies Sarah squares off against the founder of Neolution to determine the fate of her family forever. But in the aftermath, ordinary life provides its own set of challenges for the future of Sarah and her family.