Footprints lead to the breakup of a ring smuggling Chinese aliens. Published originally in 1933.

A plot to steal a top-secret instrument vital to the United States space program poses a challenging case for Frank and Joe Hardy. After rescuing a South American stowaway who mutters a warning about “Footprints “ and then vanishes, Frank and Joe discover that documents belonging to their famous detective father are missing and that footprints linger under a window. The young detective’s search for the diabolical mastermind of the “Footprints” spy ring takes them on a flight to a group of islands off the coast of South America.

Frank and Joe Hardy suspect that their best friend Chet Morton is the victim of a summer school swindle and offer to help get his money back. While probing a baffling burglary at the Seneca Indian Reservation in New York State they investigate Zoar College located nearby. A startling connection between the Zoar College swindle and the theft of the Seneca’s gold tribal relic Spoon Mouth propels the teenage sleuths into a series of perplexing and dangerous situations.

Two unexplainable plane crashes near an airport on the East Coast plunge Frank and Joe Hardy into a bizarre case. From the moment Frank and Joe find a radioactive engine in an airplane junkyard, unexpected dangers strike like lightning. Despite the repeated attempts on their lives, the teenage detectives pursue their investigation, discovering two vital clues and others that provide the solution to one of the most baffling mysteries the boys and Mr. Hardy have ever encountered.

Private investigator Fenton Hardy enlists the aid of his teenage detective sons in a search for a missing man being sought by an insurance company. All leads to the sailor’s whereabouts have petered out and the boys fly to Iceland, the man’s native land, hoping to find a new clue. From the moment Frank and Joe arrive in Reykjavik, the capital city of Iceland, they are in constant danger. In the spine-chilling pursuit that follows, Frank and Joe uncover a diabolical espionage plot that threatens the life of a U.S. astronaut and NASA’S moon project.