Sarah, Felix, and Mrs. S take a trip with the hopes of tracking down the Castor Original. But they are not alone. Back home, Donnie tries to save the family business when Pouchy defects, but it is Helena who manages to secure a “full refund”. Meanwhile, Cosima becomes suspicious of Shay, and Delphine takes the matter into her own hands. But nothing will ever be the same again when Leda finally finds what they’ve been looking for.
Siobhan, Sarah, and Felix arrive in London, to Siobhan's old neighborhood, and her pub. There they find several old friends of hers, including Terry, who has a gun that Mrs. S requested. Mrs. S knows that the only end to the Castor original is to kill him. She shows Terry the rhyme they found, "In London town", and he recognizes the number at the end: it's a prisoner number, he'll find the name.
Virginia Coady, now with her hand bandaged, does the neurological test on Rudy; he is showing symptoms, and not happy about it. The only consolation she can offer is she hopes to find the Castor original and that she has a team working tirelessly trying to decipher the Dr. Moreau book.
At the Hendrix house, Helena spends time with the children, although Gracie is often put off by Helena's violent nature. Gracie goes to a doctor's appointment.
Donnie is still at Bubbles, face covered in bruises after his fight with Jason last night. Lionel shows up to reclaim all of the drug inventory, which isn't at the store: They were only on loan while Jason vouched for them, and since Jason's changed his mind, Pouchy wants them back. He finds Helena's nitrogen tank, which Donnie doesn't recognize, and takes it; Donnie has to return all the drugs to Pouchy to get the tank back.
At Shay's apartment, Cosima looks over the dossier on Shay that Dyad produced, and is surprised to find that Shay was a soldier. She is suddenly cold to Shay, and leaves to go to Scott. They discuss Shay, and are suspicious that she's a spy, theres a lot of compelling evidence: Shay showed interest in the Dr. Moreau book; Shay mentioned the name ‘Sarah’ to Cosima, and claimed Cosima said it in her sleep, despite the fact Cosima claims she doesn’t sleep talk; Rudy mentioned to Scott that Cosima is sick and needs the original DNA, and Cosima confesses that Shay is well aware of Cosima’s illness.
Siobhan's friend Terry gets to his apartment, and sends a message to Siobhan to join him. He is interrupted by Ferdinand and his henchman, who tortures Terry for his information. When Siobhan and Sarah find him, Ferdinand is gone, and Terry is near death. He lives just long enough to whisper the prisoner's name to Siobhan, and she says they're done, nothing to do here, let's go home. Sarah quickly swipes up Terry's dropped phone off the floor without anyone noticing.
On the street outside, Sarah tells Felix they have more to do. She checks Terry's phone, finds the calls he made today, and re-calls a number. Calling herself Siobhan, Sarah arranges to meet with the person who answers.
Donnie returns home to find the drugs, and Helena notices his bruises. She gets the truth out of him, including that Lionel got Helena's nitrogen tank. She explains it: the Johanssen's took eggs from her, fertilized them, and put them frozen into the tank, and she wants her "babies". They go together, with the drugs, to Pouchy's.
Cosima goes to Delphine at Dyad, and both are concerned that Shay is a spy for Castor. Cosima does agree with what Delphine said earlier: she can't really quit Dyad, and they all need Castor gone. But they need to know who tipped off Castor that Scott had Dr. Moreau. Cosima tells Delphine that Sarah, Felix and Mrs. S are in London looking for Castor Original.
Donnie and Helena go to Punchy's (Helena presenting herself as Alison), and return the drugs, but Punchy and his people laugh when Donnie asks for their money back. They do give back the nitrogen tank. But Punchy says now they will only launder money, their drug business is over, and he threatens Donnie's kids. At this, Helena stops, hands Donnie the nitrogen tank, and shoves him out the door. "You should not threaten babies."
Delphine, with some guards, goes to Shay, and threatens to torture her for information on Shay's spying. Shay expresses only innocence.
Donnie is assuring Alison by phone that everything is fine, when Helena walks out of Punchy's, holding the detached paper-cutter blade, and covered in blood. She hands him a bag, saying "I got refund." At home, Helena thanks Donnie for saving her "babies". Alison gets home, and she and Donnie find out the bag from Pouchy was more than their refund, a lot more.
Gracie didn't go to a doctor's appointment, she meets Mark. From there she calls Cosima to say that she's sorry, but "they" told her that she could save Mark, if she told them about where the book is. She and Mark drive away. Cosima quickly calls Delphine to tell her: it wasn't Shay, it was Gracie. Delphine leaves Shay unhurt, but traumatized.
Terry's friend picks up Sarah and Felix, and it's Kassov, another old friend of Siobhan's. He knows the prisoner they want: Kendall Malone, a criminal charged with murder and was eventually let out on parole. He takes them to Kendall's address. After Sarah and Felix walk away, Ferdinand catches up to Kassov and kills him. Ferdinand calls Delphine to say he's following Sarah in London, and they agree they should get together.
Sarah goes into Kendall's apartment, and discovers not a man, but a crotchety old woman, who says she's Kendall Malone. Siobhan has followed Sarah, and joins her, and it turns out that Kendall Malone is Siobhan's mother. Not only that, she is the one who killed Siobhan's husband, John Sadler, with gardening shears back when Siobhan was seventeen. Kendall is more than crotchety, she is hostile and brutal, and is happy to have killed John, "he was a drunk and a pikey." So Siobhan and Kendall have that between them.
Sarah asks about Professor Duncan, and Kendall explains: the Duncans came through the prison, testing inmates, saying it was cancer research but Kendall never believed them. Ethan Duncan was looking for a donor, a male one, but found her: Kendall had absorbed a male twin in the womb when she was formed, she's a chimera, she has two cell lines in her body. Siobhan (still outraged at the killer of her husband) is ready to shoot Kendall, but Sarah realizes: Kendall really is the Castor original, and also the Leda original, they need her. They all leave, setting the place on fire as they go, to be sure that none of Kendall's DNA is left behind.