On a visit to the ruins of Madderstone Abbey, Penelope Lambe suffers a bad fall from the ancient stone steps. Before she slips into unconsciousness, Penelope manages to say, 'I saw her - It was her.' The Crockford sisters are sure that what made Penelope fall was that she saw the Grey Nun, the ghost that is reputed to walk the abbey's ruins. But Miss Dido Kent is less convinced that this was the reason for the accident, and is determined to find the real cause. Disregarding everyone else's certainty about the ghost, and endeavouring to take her mind off the financial troubles of her brothers, Dido turns all her mental energy to solving the mystery, and revisits the ruins to go over the facts of the accident, looking for clues as to its cause. But events start to seem more sinister when a human skeleton is found at the abbey. 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.