American investment banker Ben Hartman arrives in Zurich, the first time he's been back to Switzerland since his twin brother died there in a tragic accident four years ago. But when Ben chances upon Jimmy Cavanaugh, an old college friend, Cavanaugh draws a gun and tries to kill him. In minutes, several innocent bystanders are dead, as well as Cavanaugh, and Ben is on the run, plunged into an unspeakable nightmare.

U.S. Department of Justice field agent Anna Navarro is being stalked by a relentless killer, managing to survive only by a combination of luck, skill, and her own quick wits. The attacks on her are somehow related to her current assignment: investigating the sudden, seemingly unrelated deaths of a number of very old men around the world. The only thing that connects them is an ancient CIA file bearing the code word Sigma.

Brought together by accident, Ben and Anna realize that their only hope of survival lies with each other. Together they race to uncover the long-hidden diabolical secrets of Sigma - secrets that threaten everything they think they know, everything they believed about their friends and families, everything they were taught about history itself. For behind Sigma lies a vast deception that is finally coming to fruition - and the fate of the world is in their hands.