If there were a God, he would have stopped me. That's the message discovered atop an elementary school in downtown Atlanta. Across the street are the bodies of fourteen innocent men and women, each quickly and cleanly murdered. The sniper Galileo is on the loose. He can end a human life from hundreds of yards away. And he is just getting started. Where others see puzzles, Esme Stuart sees patterns, and these outside-the-box inductive skills made her one of the FBI's top field operatives. But she turned her back on all that eight years ago to start a family and live a normal life. She now has a husband and a daughter and a Long Island home to call her own, far removed from the bloody streets of Atlanta. But Galileo's murders escalate and her beleaguered old boss needs the help of his former protégée. But how can she turn her back on her well-earned quiet life? How would she ever be able to justify such a choice to her husband? To her daughter? And what will happen when Galileo turns his scope on them?

When the student is ready, the teacher appears. The only problem is, in this online classroom the students are would-be serial killers eager to learn the tricks of the trade from a master, the enigmatic Cain42. FBI consultant Esme Stuart is struggling to stanch the doubt and fear eating away at her marriage. Now a seedy true-crime writer is dredging up the deadly confrontation that nearly destroyed her. But the link between Esme's old enemy and this new predator is the key to the Bureau's manhunt. Esme knows her involvement in the case could cost her everything. Her marriage. Her daughter. Her life. But when Cain openly challenges his "students" to embark on a killing spree, she has no choice but to act—before Cain strikes another victim down…