The clone sisters who finally got together face a new threat. Just as Sarah and her companions began to piece together the complex conspiracy behind the shadowy Dyad Institute, they discovered the "Castor Project". This is a top-secret military project that is creating well-trained male clones. Under the watchful eye of the ruthless Dr. Coady, each of the clones of the "Castor Project" is a dedicated and deadly soldier. But who created them, and more importantly, for what purpose? Sarah's Ukrainian twin Helena struggles in the Castor cell, feeling abandoned by her newly reconciled family. Meanwhile, Alison starts a lucrative but sinister new business, and Cosima discovers the key to the synthetic sequence that could save everyone...