National Security Agency, Fort Meade, Maryland: when the most powerful intelligence organization on earth's invincible code-breaking machine encounters a mysterious code it cannot break, it calls for its head cryptographer, Susan Fletcher, a brilliant and beautiful mathematician. What she uncovers sends shock waves 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.

Seville, Spain: the creator of the code, Ensei Tankado, is found dead. And with him has died the secret to an impregnable code-writing formula that threatens to obliterate the post-cold war balance of power. Forever.