When a priest from a Catholic boarding school suddenly disappears, Humphrey and Esther find that his case has uncanny similarities to a local myth - 'The Devil On the Rocks'. Why was he last seen marching on a nearby beach in the middle of a dense fog, and where is he now? With the school's headmistress reluctant to aid the investigation, a large sum of money missing from the school safe, and the priest nowhere to be found, the team have their work cut out trying to pin down the truth.