She had once said that taking the codebook had been like tossing a stone into the ocean, the ripples of it had seemed endless. Now, some sixty years later, time finally catches up with Bletchley Park veteran Elsie Sidthorpe, as she is tested, once again, by Gunther Moller, the renegade German with his own hidden past. But this time, Elsie has an insurance policy. Or two, in fact. When the first of them fails, Bletchley curator Harry Stammers is thrust into a desperate race to unravel her secrets and the truth about a mother who has been missing since his childhood. With the mysterious Smith manipulating events for his own covert ends and Moller preparing his final move, Stammers and the acid-tongued Jane Mears will need to accomplish the impossible. But not before they have deciphered the secrets of BUILDING 41."