Sarah is on the run from a bad relationship when a lookalike stranger commits suicide right in front of her. Sarah sees a solution to all her problems by assuming the dead woman's identity and clearing out her bank account. Instead, she stumbles into a thriller mystery, and uncovers an earth-shattering secret: she is a clone. She learns there are more like her, genetically identical individuals, nurtured in wildly different circumstances, and someone...

After the shocking discovery of Project Castor, Sarah and her sister clones are left to battle the top-secret military project producing these lethal male clones. At the same time, Sarah tries to track down her missing twin Helena and an ailing Cosima searches for the key to the synthetic sequences that may keep them all alive. But how far are they willing to go to protect each other and their families, and who can they really trust? Orphan Black...