From the author of the #1 bestseller The Girl in the Spider's Web--an electrifying thriller that begins with Alan Turing's suicide and plunges into a post-war Britain of immeasurable repression, conformity and fear >But Leonard Corell, a young detective constable who once dreamed of a career in higher mathematics, suspects greater forces are involved. In the face of opposition from his superiors and in the paranoid atmosphere of the Cold War, he inches closer to the truth and to one of the most closely guarded secrets of the Second World War--what was going on at Bletchley Park. With state secrets swirling in his mind and a growing fear that he is under surveillance, Corell realizes that he has much to learn about the dangers of forbidden knowledge.