May 1941. One of the senior choristers of St Paul's Cathedral choir, Dr Edwin Roberts, is found beaten to death in the crypt of the Cathedral. Coburg and Lampson are called in to investigate, and when they get there, Coburg is pleasantly surprised to be greeted there by Arthur Waterson, now sporting a dog collar, someone he was at Eton with many years before. Waterson believes that there may be a connection between the very precious books that are houses in the Crypt and Dr Roberts' work at Bletchley Park. As Coburg and Lampson work to solve a very puzzling case, it becomes clear that there is more to the brutal murder than they first suspected.