When Mr. Hardy disappears while investigating a mystery surrounding a vacant house rumored to be either haunted or an abode for criminals, Joe and Frank search for the truth.
When Frank and Joe try to get hold of a top secret invention, they are put on the trail of jewel thieves and smugglers. Joe is kidnapped and Frank starts off on the chase.
In solving the mystery of two medallions missing from an inherited curio collection, the Hardys wind up in a desolate area of Guatemala at the mercy of dangerous thugs.
While searching in Iceland for a missing sailor sought by an insurance company, the teen-age detective brothers uncover a plot threatening the life of a United States astronaut and the secrets of NASA's moon project.
When thieves hijack a collection of stuffed animals from a country auction, Frank and Joe Hardy pursue the getaway car and are drawn into a thrilling mystery.