The Reverend Justin Somerton (Michael Bryant), a professor of Medieval History, is approached by a former pupil - Lord Peter Dattering (Paul Lavers). Dattering fears that his widowed mother is being exploited by a couple of charlatans posing as mediums and offering her the chance to commune with her late husband. Somerton agrees to attend one of their séances and outwits the fraudsters. Somerton tells Dattering of the research he has been conducting into the history of a monastery and shows him a book which details the exploits of a former Abbot - and of the treasure he is reputed to have hidden somewhere within the catacombs of the monastery. The two find a number of clues to the location of this treasure, not just in the Latin texts but in a stained glass window. They ignore the warning that the treasure is protected by a guardian - shaking Somerton's rational beliefs to the core.