RCS Hutching

The discovery of incomplete skeletal remains on a Swiss mountain by Celeste Palin's Grandparents provides Anna and Martin Price with an apparently insoluble mystery when there are no obvious clues to the person's identity. The remains had stayed hidden in a gully for almost one hundred years, and it was unclear how they came to be where they were found. Could there be any connection with the events at a British POW camp at Freiburg during 1915? Cavalry Captain Courtenay Mallory Rushwell becomes deeply involved with a German woman working for Abteilung iiib, the Imperial German Intelligence Service.
Exactly which of them is a traitor to their country?