Castor pursues the Leda clones for original tissue samples. Helena, who is pregnant and imprisoned, is examined by a mysterious new villain. The Castor boys are also out to play, toying with a frightened young woman, Patty, in a sexy switcheroo that ultimately sets Sarah – and Art – hot on the Castor trail. But it’s not all fun and games for the Castor boys, as Sarah gleans they may have a mysterious defect that puts their lives in peril.

Mark is taken down in a Prolethean shoot-out and Sarah must step in to help. But as Sarah gathers intel that puts her on the trail of the original Castor DNA, a familiar foe is not far behind. While Helena continues to toil in a faraway military compound, it is ultimately Sarah’s fate that will hang in the balance as she tries to save her sister. Meanwhile, Gracie is cast out from the Prolethean fold, and Alison and Donnie come face-to-face with unhappy drug lord, Jason Kellerman.

Helena and Sarah are reunited at the Castor camp. As Sarah suffers testing, Paul appears to chide her; but his allegiances are beginning to strain. While the Leda family worries about Sarah’s absence, Gracie ‘breaks loose’ under the care of Mrs. S and an unwilling Felix. Cosima falls hard for an internet date, Shay.

Paul digs and finds the ugly truth behind the Castor Boys’ logbooks of names, pushing him into action. Meanwhile, Felix is desperate for info on Sarah’s whereabouts, and presses the still-ailing Rachel for any shred of a lead. In order for Donnie and Alison to take their money-making endeavor to the “next level” she must introduce Jason Kellerman to her unique business plan.