On a visit to the ruins of Madderstone Abbey, Penelope Lambe suffers a bad fall. The Crockford sisters are sure Penelope must have slipped when she saw the Grey Nun, the ghost that is reputed to walk the abbey ruins. But Miss Dido Kent is less convinced that this was the reason for the accident, and is determined to find the real cause. Then events take a sinister turn when a human skeleton is found nearby. The remains are identified as those of a Miss Elinor Fenn, and letters come to light which hint at the reason for her death. But how is Miss Lambe's accident connected to this discovery? Did she see a ghostly warning? Or is there a rational explanation? Everyone is relying on Dido to find out.