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.