Nancy and her friend Helen visit their friend Emily Willouby at the Lilac Inn, which Emily now owns, to help her plan her wedding. Emily plans on selling inherited diamonds in order to help fix up the Lilac Inn. However, Nancy soon learns that someone has been impersonating her and making expensive purchases under her name. Soon after, Emily's diamonds are stolen! Can Nancy find the thieves and recover the missing diamonds?

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...

Nancy encounters a mysterious saleswoman in a perfume shop, a curious man on the train, and then finds herself trying to decipher a strange code.