This is the account of how, for four years, British and Allied codebreakers decrypted secret SS and Gestapo messages detailing the mass killings of the Holocaust, and how the Germans in turn deployed cryptanalysis to try to conceal their persecution of Europe’s Jews. The compelling and fast-paced narrative is told from the perspectives of two central and opposing characters, who never meet. At Bletchley Park, there is the legendary but unsung British codebreaker Nigel de Grey, shy, determined, nicknamed ‘the Dormouse’ by his colleagues.