Nancy and her friends witness an explosion and the burning of a beautiful country mansion. Fearing its occupants may be trapped in the blazing building, they rush to the rescue and unexpectedly find themselves confronted with a mystery that seems insoluble. The first clue is an anonymous diary, its entries in a handwriting difficult to decipher. Who dropped the diary? Was it the stranger Nancy saw running away from the fire? What was he doing there? Finding out how Nancy discovers the answers to these questions makes for another exciting Nancy Drew mystery.