When the NSA's invincible code breaking machine encounters a mysterious code it cannot break, the agency calls its head cryptographer, Susan Fletcher, a brilliant and beautiful mathematician. What she uncovers sends shockwaves through the corridors of power. The NSA is being held hostage...not by guns or bombs, but by a code so ingeniously complex that if released it would cripple US intelligence. Caught in an accelerating tempest of secrets and lies, Susan Fletcher battles to save the agency she believes in.