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.
Felix and Sarah manhandle Seth's body into Felix's bathtub, it's a disgusting job, and they wonder what to do with it. Art shows up and wants in, though when he finds the body, he accepts their explanations that it wasn't them. They call Cosima for advice, which is Art's first introduction to her. Cosima and Scott know the Castor clones have some design flaw, likely neurological, and want to examine Seth's brain. Art has a lead on Mark.
Mark and Gracie are in their motel room, and Mark is worried that her family will be looking for them, so he says they are laying low. She says they should be enjoying their honeymoon, to which he agrees. After some sex together, Mark says when he was in military academy, the other guys were into sex, but Mark was waiting. And though he told her father he was AWOL, he's not, he's on assignment to recover some scientific material her father had, and he's sure it was kept by Willard Finch who lives nearby.
Alison and Donnie are on the sidewalk, greeting neighbors and handing out campaign flyers for Alison as school trustee. They come across a soccer mom, who says she isn't interested in someone with Alison's "issues" handling her children; as she turns to leave, Alison tells her she has taken over Ramon's business, and offers her a box with soap and bath beads, and lifting up the soap Alison reveals the strip of drugs beneath. The mom must be one of Ramon's customers, now Alison's, and she pays and accepts a flyer, and now she's willing to talk about the school board.
Art shows Sarah pictures he took of the Johanssen ranch, and they know the family scattered after the fire, but they're looking for Castor samples. They see Alexis, one of the Johanssen family, and ask her about Helena, but Alexis doesn't know where she went, although she knows Helena is pregnant with one of her eggs fertilized by Henrik. She also knows Mark fled with Gracie, and Art gets a police report of where Mark's truck went.
Gracie and Mark discuss Willard Finch, and if he was hiding something for Henrik he wouldn't just hand it over. Gracie wants to avoid violence, and offers to get it. She goes alone to Finch at his farm, and he knows her, and is softened enough by her to let her take the crate that Henrik left with him.
At the desert military base, a truck brings Paul and Rudy in, and Dr. Coady immediately meets them and slaps Rudy, saying he killed his brother. Rudy protests, "he was stage five in the field", and killing such brothers is their protocol, but she says it wouldn't have happened if he and Seth had followed orders and gotten back to base.
Paul says the "defect" is getting worse in the boys in the field, Seth went from asymptomatic to "stage five" in a day. Dr. Coady says they did the stress test, the "NeuroCog", on Helena, and she doesn't have the defect, so her biology might have answers. It appears Paul is the program's chief of field operations, but Dr. Coady is program manager, and she sends Paul to Arlington to report to the Director, they could get shut down.
Cosima and Scott dissect Seth's body in Felix's bathtub (well, Cosima does it, Scott tries not to throw up), much to Felix's disgust. But they get the brain cut out and stored for analysis.
Dr. Nealon is with Rachel, who has a bandaged head and is in a motorized chair, is performing exercises with flashcards and is having trouble finding words to say and pronouncing them. But he says her aphasia and motor skills are actually improving. She stammers and points to the door, perhaps to ask "can I leave", and Dr. Nealon says that, sadly, Topside has been told that she died in a plane crash, and Delphine is in charge. She asks "I finished?", and he says she is still valuable, they will focus on her recovery, and then flashes her a card of the two-headed horse CASTOR symbol, which Rachel simply recognizes as ‘horse’.
Dr. Coady meets Rudy in her office; for Major Paul he only pretends to stand at attention, but for "Mother" he is scrupulous. She asks for his "logs", and he gives her the little notebooks that he and Seth had; she looks through them a bit, they seem to be the ones that Patti reported they wrote all of her ID into, along with a bit of her hair. Rudy says, yes, he should have come back to base, but if there was any chance Sarah had the Castor samples, he had to try.
Alison leaves her house, and some neighbor mothers say they've heard of her new business, and would like to buy some ... soap? Alison offers a special for new customers, and gives them sample boxes (soap, with drugs inside). She notices Marci Coats campaigning nearby, and mentions the election to her customers.
Alison and Donnie are at work in their garage, making soap and boxing it with drugs. Marci knocks on the door, and they hide the drugs; Marci has a proposal for them, a house she can offer a good deal on, but they realize it's in a different school district, and aren't interested. Marci tells them Alison can't really win, and leaves.
Art and Sarah visit Willard Finch at his farm, and ask about Mark, or Gracie, but he won't tell them anything, though they threaten him a bit with his past running guns for a militia.
Alexis comes to a house in the country, and Bonnie Johanssen answers. Bonnie says Alexis is banished, but Alexis thinks she knows where Mark took Gracie.
At the motel, Mark opens the crate that Willard had, and finds nothing inside but notebooks and journal articles, no samples. He's sure Willard has more, and goes to threaten him. He sends Gracie to a diner to wait for him.
Art and Sarah are eating at a diner, and Sarah says Art has real police work to get back to, he should go back. Art spends a while explaining how close he was to Beth, and Sarah is as "fierce" as Beth was, and needs Art's help too. But he consents and leaves.
Sarah looks around, and discovers Gracie eating there too. She sits with Gracie, and asks about Mark and his brothers, which surprises her: "my husband doesn't have any brothers." Sarah tries to assert a connection between her and Gracie, to get some cooperation, but doesn't get anywhere. She shows Gracie a picture of Rudy and Seth, and that tells Gracie what Mark really is. Sarah asks where Mark is.
Mark is at Willard Finch's farm, torturing Finch for more information, trying to find the samples, but Finch dies from the torture.
Gracie goes to the motel and packs, and Bonnie walks in. Bonnie is entirely unsupportive of Gracie's escape with Mark, and it seems Gracie's only value is that she is pregnant with Helena's child. She says they have a "purpose", and takes Gracie.
At Dyad, Scott and Cosima examine cells from Seth's brain, and find well advanced illness. She cross references Seth's DNA against the Leda DNA, and finds something important enough to call Sarah about. Sarah is outside the Finch farm, but takes the call: there are substantial blocks of matches between the Castor and Leda DNA sequences, the only possible meaning is, "the boy clones are our brothers".
Sarah finds Finch dead, and Mark with him, looking a bit remorseful, he planned on Finch talking, not dying, and he miscalculated. She says they are doing what they can to get Helena back, which Mark doesn't care about. Sarah says he should, they are his sisters. He doesn't accept her as family, and only cares about Gracie; he leaves to his truck, when a gunshot is heard, and he falls, and crawls off to the cornfield. Bonnie shows up with a rifle, follows him into the cornfield, we see her aim, and we hear another gunshot.
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