 During the heart of World War II, in March of 1943, cryptoanalysts at Britain s code-breaking center have discovered to their horror that Nazi U-boats have changed their Enigma Code. Authorities enlist the help of a brilliant young man named Tom Jericho (played by Dougray Scott) to help them break the code again. The possibility of a spy within the British code-breakers  ranks looms and Tom s love, Claire (Saffron Burrows), has disappeared. To solve the mysteries, Tom recruits Claire s best friend, Hester Wallace (Kate Winslet). In investigating Claire s personal life, the pair discovers personal and international betrayals.