Siobhan, Sarah, and Felix arrive in London, and they make contact with one of Siobhan's old friends. He gives her a gun, and she tells the kids she plans to kill the Castor original. 

In Mexico, Rudy starts displaying symptoms of the Castor defect.

Siobhan's contact recognizes the number at the bottom of Rachel's riddle as a prisoner tag from back in the 80s. They realize that the Castor original is in prison.

Donnie recovers from his fight with Jason, but Helena sees his injuries. He tells her Jason took her frozen embryos from Bubbles, and she swears to find him.

Cosima looks through Shay's file that Delphine collected, and she realizes that she's former military. She and Scott talk about the possibility that Shay is a Castor spy, considering she asked about Sarah and found Duncan's book. 

Siobhan reconnects with her old band, and Sarah and Felix urge her to perform. Her contact tells her he's found something, but Ferdinand shows up to his apartment and tortures him before he can tell her. 

Gracie leaves the Hendrix's under the pretense of a doctor's appointment, but she's actually running away.

Cosima returns to Dyad to ask Delphine for help exposing Shay. 

In London, Siobhan finds her contact just before he dies, and he whispers what he found. She tells Sarah and Felix they're going home, but Sarah won't give up. She steals the dead man's cell phone and poses as Siobhan to get information. 

Donnie and Helena return the pills to their local supplier after Jason bad mouths them, but when he threatens Donnie's kids, Helena attacks.

Delphine confronts Shay, who says she doesn't know anything about Castor. 

Sarah finds out that the Castor original's name is Kendall Malone, and he's on parole. She goes to his house, only to find Kendall is a woman, and Siobhan's mother.

Gracie calls Cosima to say goodbye, and she reveals that she was actually the Castor mole. 

Siobhan's mother reveals that she absorbed a male twin in the womb, and that was the source of the Castor and Leda original DNA.