Plot summary
Art and Sarah work together to find a lead on Mark's location. Mark reveals to Gracie that he was sent by the military to infiltrate the Proletheans to find the original Castor DNA, which he believes Henrik entrusted to farmer Willard Finch for safe keeping. It is believed the DNA would be used to cure the Castor defect. Gracie recovers the box from Finch, but there is no DNA inside. Henrik's wife Bonnie tracks Gracie down to bring her back to the Proletheans. Sarah learns that Art is devoted to helping the Clone Club because he was in love with Beth.
Cosima extracts Seth's brain and learns that like the Leda clones, the Castor clones are also suffering from a defect, albeit a neurological one, as well as determining that both sets of clones are biological siblings. Alison gains support for her campaign after selling the other mothers drugs hidden in bars of soap. Nealon treats Rachel's brain damage. Rudy confronts Finch but he dies from a heart attack during the torture. Sarah's arrival is soon followed by Bonnie, who shoots Mark.
Credits
Adapted from Wikipedia contributors, List of Orphan Black episodes. HTML formatting and reference markers removed.