A disappearing floor, a savage-looking hound, a galloping ghost and a startling invention are just a few of the strange elements that complicate Frank and Joe's efforts to solve the mystery of the death of an heir to a mansion.

A banker who has been receiving threatening notes enlists the help of the Hardy boys. Before very long, the young sleuths find themselves entangled in the investigation of a notorious band of thieves.

Frank and Joe go to the Pocono Mountains to investigate the mystery of a legendary witch who stalks Black Hollow seeking vengeance. Disappearances occur and strange screams fill the night.

Determined to learn the secret of the old mill, Frank and Joe employ a clever ruse to gain entrance and become trapped. There they unravel two mysteries, one involving a counterfeiting case and the other a national security case about the missile parts that their father is working on.

Frank and Joe investigate the legend of Whalebone Island when they receive a warning concerning a sunken freighter.

Frank and Joe, the Hardy boys, solve a mystery involving a magician, American secret agents, and international spy ring, and a Scottish castle.

The Hardy boys embark on another puzzling mystery when they try to discover who is stealing rare collections of ancient pirate treasure and smuggling them out of the United States.

While their father is on vacation, Frank and Joe Hardy attempt to help Burt Worth who is being sued for libel for something he did not write.

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

When their peregrine falcon brings down a homing pigeon carrying rubies, the Hardy brothers find themselves involved with kidnappers.