Never told in detail before, 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.

In Nazi-occupied Poland, SS Major Hermann Höfle, a former taxi driver from Salzburg, and one of the Third Reich's ruthless bureaucrats of mass death, oversees the operations of five concentration camps, including Treblinka. De Grey fought hard to make sure the vital intelligence from decrypted signals reached Allied leaders and was acted on. Höfle, meanwhile, used complex coded messages to try to conceal the mass killings.

De Grey worked with his American counterparts, as well as codebreakers and intelligence agents from the Soviet Union, France, the Vatican, Switzerland and Poland. Yet he had dangerous enemies closer to home: a cabal of senior British government and intelligence officials disbelieved or ignored repeated intelligence reports about the ongoing Holocaust. Flawlessly researched, this is the story of a battle between good and evil, between life and mass death, a cat-and-mouse war of electronic wits.

More than eighty years on, as Russian leaders face war crimes charges in international courts, the words 'Never again' seem more pertinent than ever.