Retiree Oswald Gibson is shot and killed in Sussex, England while he's out fishing. The killing is similar to the recent murder of an elderly doctor in Scotland. Investigation reveals that shortly before his death Oswald wrote - but didn't send - a letter inquring about John Madden, a former Scotland Yard detective. As more elderly men are killed Detective Inspector Billy Styles of Scotland Yard asks his retired supervisor John Madden to help with the investigation.
It's soon discovered that thirty years before, during WWI, all the dead men were in the military and served in the same region of France as did John Madden - though Madden has no memory of the victims. Further investigation reveals that while in France all the men were involved in the same unfortunate army incident. Wanting to prevent further killings Styles and his team, including female detective Lily Poole, try to learn more about the occurrence, but the records are tightly sealed and unattainable.
In an attempt to discover the identify of the serial killer the detectives question neighbors, friends, relatives, and household help of the victims and slowly amass clues that help explain the killing rampage and reveal who might be involved. The murderer, though, is adept at concealing identity and hiding out, and is very difficult to catch. This leads to an engaging game of cat and mouse between Scotland Yard and the murderer, and detectives get knocked around and shot during the pursuit. During all this John Madden has quiet family moments with his physician wife Helen, sees to his farm, and helps a nonagenarian aunt with much needed house repairs - all of which provides a nice little break in the action.
Eventually all is revealed and the killer is cornered. The story brings home the horror of war and the suffering caused by post-traumatic stress disorder, which was completely unrecognized during WWI. The characters in the story are well-rounded, interesting, and believable and the story held my attention. Plus, it was good to see one of the early (fictional) female detectives hold her own at Scotland Yard. A good addition to the John Madden series.