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 fund themselves confronted with a mystery that seems to be insoluble. The first clue, an anonymous diary, includes not only indecipherable handwriting, but puzzling technical drawings and chemical formulas as well. 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? When evidence mounts against Joe Swenson, who was swindled by "Foxy Felix", our young detective makes the desperate effort to exonerate the inventor of the suspicion of arson because of his dear five-year-old daughter.