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Based on what Rachel had deciphered of Duncan's code, Sarah, Felix and Mrs. S believe the answer to finding the Castor original is in London, where they head to Mrs. S' old stomping grounds prior to Sarah and Felix entering her life. Help from Mrs. S' old network of friends doesn't turn out quite the way they expect, which leads to diverging opinions on what to do next. Cosima now believes what Delphine had intimated, that Shay is a mole for Castor. Cosima and Delphine individually have to decide what to do based on that belief. Donnie's physical altercation with Jason jeopardizes the business partnership between Jason and the Hendrixes, with the resulting actions profoundly affecting Helena. Helena proves to be much more resourceful in resolving the matter to her satisfaction than Donnie imagines.
- Writers: John Fawcett, Graeme Manson, Alex Levine, Kim Coghill, Andrew De Angelis, Jeff Detsky, Adam Higgs, Bob Mackowycz Jr., Elize Morgan, Peter Rowley, Rebecca Sernasie
- Stars: Ari Millen , Dylan Bruce , Evelyne Brochu , Jordan Gavaris , Josh Vokey , Kevin Hanchard , Kristian Bruun , Maria Doyle Kennedy , Skyler Wexler , Tatiana Maslany