Ultra was the most effective system for reading the enemy's mind ever employed in war. By enciphering a multitude of high-level signals on their complicated Enigma machine, the Germans thought they were invulnerable. But they were broken by Station X at Bletchley Parl. Here was the source of Ultra, the `special intelligence' organization which was the best kept secret of the war. Books, articles and programmes on radio and TV have touched on aspects of Ultra, but the need is great for an authoritative, panoramic view. Lewin tells the full story in fascinating detail, throwing fresh light on many campaigns....