Sarah repels a potentially lethal Topside menace as she fights to locate a disappeared Helena, and the threat of Castor looms anew.
The war is all but lost when Rachel's latest plan forces Sarah to concede. Sarah uncovers a terrifying dimension to the clone conspiracy.
Castor pursues the Leda clones for original tissue samples; Helena, who's pregnant and imprisoned, is examined by a mysterious new villain.
All sides are in hot pursuit of original clone DNA until a shocking revelation about the nature of the Castor-Leda connection stops everyone dead in their tracks.
On the season finale, Castor and Topside force Sarah to make uneasy alliances and the results of Alison's candidacy finally come in.