Nancy and her friends, George and Bess, are returning from a country carnival when they witness the explosion and 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 to be insoluble.
The first clue is an anonymous diary, its entries in a handwriting difficult to decipher. Even more puzzling are the technical drawings and chemical formulas contained in its pages.
Who dropped the diary near the burning house? Was it the gaunt stranger Nancy glimpsed running away from the flaming structure? What was he doing there? And does he know the whereabouts of Felix Raybolt, an unscrupulous dealer in patents, who has not been seen since his home burned? Or did Raybolt die in the fire?
Fire investigators believe that the destructive explosion and fire may not have been an accident — but an act of revenge. Evidence mounts against Joe Swenson, an inventor who was swindled by “Foxy Felix.” Prompted by her affection for Mr. Swenson's five-year-old daughter Honey, the young detective makes a desperate effort to exonerate the inventor of the suspicion of arson. How she accomplishes this makes another exciting Nancy Drew mystery.