Teenage detectives Frank and Joe Hardy investigate a supposedly haunted house reported to be the hangout of criminals.

"Drop case or else danger for you and your family." This warning addressed to Fenton Hardy not only alarms the famous investigator and his detective sons, but also puzzles them, because Mr. Hardy is working on more than one case. Which one prompted the threat? When more ominous warnings follow, Frank and Joe suspect there is a link between the counterfeiting case they are investigating and a secret case their father cannot discuss since it involves...

As Frank and Joe investigate the mysterious disappearance of a college professor, they stumble upon a baffling secret in a complex network of caves.

The Hardy brothers interrupt their investigations of jewelry-store holdups to answer a plea from their cousin on a New Mexico cattle ranch.

The Hardy boys and two friends take a camping trip to the Rocky Mountains in an attempt to locate a gang of credit-card counterfeiters.

The search for a stolen boat leads the Hardy brothers to Mexico where they become involved with a band of Indians and a strange smuggling operation.

Helping in a case their father is working on, the Hardy brothers try to find an informer who has telephoned from a booth in their home town, and are hired by a carnival owner to spot pickpockets.

On a winter vacation in Jamaica the Hardy boys begin a dangerous adventure when an ancient bronze death mask is discovered near their beach house.

When the Hardy boys eagerly agree to assist their friend Jack Dodd and his father in locating a family treasure, the two young detectives are unaware of the baffling complications that will arise. The only clue to the long-lost treasure is a cryptic message written by a Pilgrim ancestor. But when Mr. Dodd and Jack are accused of stealing cars and then mysteriously disappear, Frank and Joe are confronted with a triple mystery. Have their friends run...