Quinn Mitchell is a nine-to-five spy--an intelligence analyst for the CIA during the day, and a suburban wife and mother on evenings and weekends. After her young daughter is killed in a tragic accident, sending her life into a tailspin, Quinn hopes to find a new start in her latest assignment: investigating a series of bizarre international assassinations whose victims have been found with numeric codes tattooed, burned, or carved into their flesh. As Quinn follows the killer's trail across the globe, always one body behind, she begins uncovering disturbing connections between the murders--and herself.

Every lead she tracks down in pursuit of the assassin brings Quinn one step closer to the Epoch Index, a mysterious encrypted message discovered in the archives of the Large Hadron Collider. Its origins are unknown and decrypting it is beyond even the CIA. Yet nothing else can possibly link together a slew of unsolvable murders, an enigmatic and sophisticated serial killer who always seems to be three steps ahead, a quirky young physics prodigy whose knowledge extends well beyond her years, and, underlying everything, the inescapable tragedy of Quinn's own past. Discovering the meaning of the Epoch Index leads Quinn to a shocking twist that shatters everything she thought she knew about the past, the future, and the delicate balance of right and wrong that she must now fight to preserve.