The problem facing Masters and Green, a team of top detectives at Scotland Yard, is a peculiarly taxing one Green receives an unsigned letter from someone who claims to have served with him in the Artillery during the war, and who reports that he has just witnessed a murder He dare not go to the local police, because one of the murderers was a senior member of the local force In fact he's not putting his life at risk by revealing his identity to anyone He offers no more than roundabout hints and cautious clues