At a school benefit sale, Pete Hollister buys an old book about early New York tunnels. Tucked in the pages is a message written in Chinese. The only English word in it is "Help!" The Hollisters take the mysterious note to a Chinese friend for translation and learn that the letter was written many years earlier in China by a Yuen Foo to his son in New York, but was never mailed. Pete, who has been trying out a new detective kit, tests the letter for possible invisible ink and is rewarded when concealed Chinese writing shows up. It says Yuen Foo was being threatened because of the "treasure." Mr. Hollister has been invited to New York as a consultant to a toy manufacturer who is trying to perfect a toy called the "Soaring Satellite." Mr. Hollister decides to take the family with him for a vacation. The children are pleased not only because of the things to see in New York but because the clues point to New York's Chinatown as the location of and answer to the mystery surrounding the letter. This book is a tour of one of the most famous cities in the world as well as an introduction to a little piece of China and the Chinese way of life at the tip of Manhattan.