Ben Hartman is vacationing in Zurich, Switzerland when he chances upon his old friend Jimmy Cavanaugh--a madman who's armed and programmed to assassinate. In a matter of minutes, six innocent bystanders are dead. So is Cavanaugh. But when his body vanishes, and his weapon mysteriously appears in Hartman's luggage, Hartman is plunged into an unfathomable nightmare...Meanwhile, Anna Navarro, field agent for the Department of Justice, has been asked to investigate the sudden, random deaths of eleven men throughout the world. The only thing that connects them? A secret file, over a half-century old, that's linked to the CIA--and is marked with the same puzzling codename: Sigma. As Anna follows the connecting thread--and Hartman finds himself on the run--she ends up in the shadows of a relentless killer who is one step ahead of her...victim by victim. Now, she and Hartman together must uncover the diabolical secrets long held behind Sigma. It will threaten everything they think they know about themselves--and confirm their very worst fears...