Plot summary (original edition)

The boys help to bring a gang of bank robbers to justice.

Plot summary (revised edition)

The Hardy boys are asked by their father, Fenton Hardy, to assist him in solving a case involving a notorious jewel thief and his accomplices. Initially, they spot the man’s car and attempt to follow it until the car drops a purple smoke bomb and the Hardys are forced to pull over. While returning home from the car chase, they run into their friend Chet Morton who was in the woods rock collecting when he heard weird screams. When they go to investigate, they find a strangely tiled floor in the woods and nearby an injured man suddenly revives and runs off. Their adventure continues with a stolen amethyst, a bomb damaging their boat the Sleuth, and a savage looking hound which turns out to be an electrified scare device.

When Aunt Gertrude hears where the boys found Chet, she tells them that they were close to Old Man Perth’s house where strange deaths have happened. The owner, Jerome Perth, was a business tycoon who made many enemies due to his swindles. When he died his nephew, Clarence Perth, inherited the house. Shortly after moving in, the servants awoke during the night to him screaming and he was found on the floor of his bedroom with a fractured skull. The door and the windows of the room were all locked from the inside, so the mystery of how he died was never solved.

The Hardy boys identify a pilot who may be helping the thieves, so they walk into his hangar to examine his plane. Armed with a miniature camera, Joe snaps a picture of a map found in the plane. This leads them to a cabin hidden in the woods in a small bight named Tigers Bight. Unfortunately, the boys don’t have a chance to investigate the cabin as their family friend Jack Wayne crashes his plane Skyhappy Sal nearby and is injured. After rescuing Jack, the boys return to the Perth mansion where they find the jewel thieves hiding out and tricking a professor into making a ‘freeze ray gun’ for them.

After more investigating, the Hardy boys manage to figure out that Clarence died by accident due to the 'disappearing floor' in his bedroom. Unfortunately, the boys are captured before they can tell any one, but luckily the jewel thief likes to brag and tells them all about how the gang has used the freeze ray guns to stun security guards and steal the jewels. He even tells them about a swindle where they rigged an elevator to go to the wrong floor where they had set up a fake office in order to receive a special shipment of diamonds without anyone becoming suspicious until they were safely far away. Eventually Fenton Hardy shows up and rescues his boys and arrests the thieves.

Original edition (1940)

• Ghostwriter: John Button
• Summary: The boys help to bring a gang of bank robbers to justice.

Revised edition (1966)

• Revision type: Completely rewritten; some plot elements retained.
• Ghostwriter: James Duncan Lawrence