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.

The Hardy Bous #48.

During Mr. Hardy’s investigation of a ruthless gang of terrorists, Frank and Joe witness an explosion in the sky near an airborne dirigible owned by Quinn Air Fleet. The young detectives look into a clue that takes them into a new animal park outside Bayport, where they are lured into a trap by an unknown enemy. Problems arise for the park owner as he receives pressure from a competitor and a real-estate firm to sell out. Strange occurrences at the park also frighten the visitors and animals. Frank and Joe take up the case despite fore warnings.

When a man selling leather goods door-to-door steals the key to their detective father’s file cabinet, Frank and Joe Hardy set out to track him down. An odd mark on a key case which the man sold to their mother leads the teenage sleuths to an Indian village, whose chiefs begs them to help him find valuable tribal possessions and the deed for his land. How Frank and Joe find the missing deed and Ramapan treasures, how they prevent the phony leather-goods salesman from carrying out a ruthless scheme and how they help their father solve the top-secret case he is working on for the U.S. government makes exciting reading for all fans of the Hardy boys.

Once again Frank and Joe Hardy take on a puzzling case when their famous detective father asks the boys to assist him in tracking down a notorious jewel thief and his accomplices. The trail leads to the outskirts of the Hardys’ home town and to a weirdly guarded mansion on the old Perth estate. With their pal Chet Morton, the brothers must tackle the mystery of the mansion heir’s sudden death. A disappearing floor, a huge, savage-looking, hound, a galloping ghost, and a college professor’s startling invention are just a few of the strange elements that complicate the boys’ efforts to solve both mysteries.

A dying criminal confesses that his loot has been stored "in the tower." Both towers of the looted mansion are searched in vain. It remains for the Hardy boys to make an astonishing discovery that clears up the mystery and clears the name of a friend’s father.