To secure Dyad's help in locating Helena, who has been abducted by Castor, Sarah agrees to impersonate Rachel to try and fool an inquisitor from the Topside cabal. Cosima has a tearful reunion with Delphine, and Mrs. S is attacked by someone looking for Professor Duncan's research.

Detective Bell brings Sarah a lead about two of the Castor clones, who are searching for the original Castor source material. Cosima's condition shows significant improvement, and one of Helena's captors attempts to gain her trust.

Art brings Sarah a lead on Mark, the Prolethean Castor clone, who is searching for Castor tissue samples with his new wife, Gracie. Cosima and Scott try to learn more about the Castor's biology, while Alison and Donnie launch their new business with great success.

Sarah tries to help Mark stay alive after he is shot by the Proletheans, while Helena begins to plan her escape from Castor's military compound. Alison and Donnie find themselves under the thumb of a drug supplier, and Felix tries to help Cosima move on.

Sarah awakens to find herself a prisoner at the Castor base, where Helena is convinced that her sister has sold her out. While Cosima has her first blind date, and Gracie sets about reinventing herself -- with help from Felix and Mrs. S -- after being expelled by the Proletheans.

With an ailing Sarah being held by Castor, Paul tries to get to the bottom of Dr. Coady's illicit experiments. Delphine returns with questions about Cosima's work, and Donnie and Alison approach Jason Kellerman about taking their business to the "next level." Meanwhile, Felix confronts Rachel for information about Sarah's location.

As her campaign swings into high gear Alison must convince her mother to sign over her store, while Jason takes Donnie to meet a powerful drug lord -- where things don't go entirely according to plan. Scott has a face-to-face with Rachel, and Helena confronts Mrs. S.

Sarah agrees to talk to Rachel face-to-face, and is forced to do her sister a favor in order to secure Duncan's clone research. Helena and Gracie settle in with the Hendrixes, and Alison confronts Jason about their relationship. Meanwhile, Felix must go undercover to introduce himself to another Leda clone.

Sarah, Felix, and Mrs. S travel to London in search of the Castor Original, but soon find themselves in over their heads. Donnie tries to salvage the family business after Jason turns against him, and Cosima takes her suspicions of Shay to Delphine.

Sarah makes uneasy alliances in order to prevent Castor from acquiring the samples that could save their line, as Mrs. S has a reckoning with her mother. Alison learns her election results, and after learning who is pulling the strings of both Castor and Leda, Delphine tries to make things right with Cosima.