The book reveals the story of British Codebreakers from the reign of Elizabeth I to the Cold War. It explores the use of ciphers during the Napoleonic wars, the role of the Royal Mail's Secret Office and the activities the Admiralty's Room 40' leading to the creation of the Government's Code and Cypher School. The main theme of the book are the events of the Second World War and the battle to break the German enigma codes. The centre of Britain's codebreaking operation was located at Bletchley Park in rural Buckinghamshire and it was from here that a hastily assembled army of codebreakers battled to decipher Nazi German's secret wartime communications. The deciphered high-level signals intelligence was known as Ultra and had a major influence on the outcome of the war, most notably contributing to crucial successes in the battle for the Atlantic and the D-Day landings in June 1944. The book also reveals the work undertaken in the Far-East and the allied efforts to break the Japanese military cipher code named Purple. The book ends with a re-assessment of the work undertaken by the British code breaker and mathematician Alan Turing and a brief overview of the codebreaking operations undertaken by GCHQ during the formative period of the Cold War.