Danger in the Fourth Dimension

Frank and Joe are asked by their father, Fenton Hardy, to meet him at the Fourth Dimension Theme Park in South Carolina to help him with an investigation he's conducting at the park. The arrangement is for Frank and Joe to fly there, check in to the hotel on the park's premises, and meet their father, Fenton Hardy, the following morning at the hotel restaurant for breakfast.

When Frank and Joe go to the hotel restaurant to meet their father, but Fenton Hardy ends up missing, and his sons, Frank and Joe set out in search of him. They soon figure out that he's been kidnapped. They aren't sure who kidnapped their father, or what the investigation is even about. Starting from scratch, they start investigating while working undercover as interns for the park. But somebody knows exactly who they are, and they end up in numerous dangerous situations, as well as receiving threatening notes and threatening phone calls.

Every employee is a suspect, including the park's owner. Eventually, through extensive investigating of their own, Frank and Joe figure out the mystery their father is investigating, where their father is being held prisoner, and figure out the suspects' identities.