There's a plot to steal a top-secret instrument being developed by the United States in a factory near Bayport and Frank and Joe Hardy are in the case up to their necks. Along with Chet, of course!

Unfortunately the boys' father, Fenton, is absent on an important case of his own.

It starts with the two heroes rescuing a stowaway and his strange utterance about "footprints." But he flees. It appears to be an espionage ring, but they can't be certain. At one point the teens, along with pilot Jack, follow the clues to a group of islands off the coast of South America!

Librarian's note: this is an introduction to a later edition. The original 1933 edition under the same title relates to a Chinese laundry in Bayport and human smuggling - with racist overtones.