George Felse is Ellis Peters' modern-day detective (the series was written in the 1970's), compared to her Brother Cadfael mysteries.
In this story, George is investigating the disappearance of an obstreperous schoolboy in a Roman archaeological site on the border of England and Wales. Also at hand are Charlotte Rossignol, the great-niece of one of the archaeologists who has written about the site, and a mystery man called Gus Hambro, who is very knowledgeable about Roman remains. Unfortunately, shortly after the boy's disappearance, Hambro is attacked and pushed into a flooding river to drown - fortunately, Charlotte is around in time to rescue him. The mystery then hinges on who has caused the schoolboy to disappear, Hambro to be attacked, and is there any link to the strange disappearance of Charlotte's great-uncle, Alan Morris?
I must admit I began to have some idea before the end as to "whodunnit", but I never thought of the whole ramifications of the plot!