Plot summary (revised edition)

The book begins with Chet Morton showing off his new metal detector to the Hardy boys and Biff Hooper while inviting them to camp at Honeycomb Caves. Meanwhile, their father, Fenton Hardy, is working to protect a Coastal Radar Station from sabotage during its construction. They are interrupted by Mary Todd who tells them that her brother, Morgan Todd, is missing and asks Fenton to find him. The Hardy boys and their father decide to team up to both find Morgan Todd and protect the Coastal Radar Station.

The Hardy boys travel to Kentworth College and meet Todd's colleague, Cadmus Quill. A clue leads them to Rockaway, but when it is mentioned they notice strange behavior from Cadmus Quill. While driving to Rockaway they hear a radio report that the radar station has been damaged, so they instead return to Bayport. Their help is not needed so they leave for Rockaway, stopping at Palis Paris to purchase a spinning wheel for their Aunt Gertrude. When they stop in at Tuttle's General Store Mr. Tuttle warns them to keep away from Honeycomb Caves because people have seen strange lights and heard shooting coming from the caves. Instead of leaving, the boys decide to camp at Honeycomb Caves with their friends Chet and Biff.

At the caves they meet a strange hermit who invites them for breakfast, then chases them off and even shoots at Frank. Their adventure continues with Callie Shaw, Iola Morton and Mary Todd trying to get jobs at the Palis Paris, Biff getting knocked out while waiting in the parking lot, Chet's metal detector suddenly exploding and the Hardy boys' boathouse catching on fire.

The story concludes with the Hardy boys finding a submarine delivering supplies to the hermit in the caves. They explore the cave and learn that the caves have an underground passage to Palis Paris where a device was being built to interfere with the new Coastal Radar Station. The Hardy boys trap the criminals, including Cadmus Quill, in the cave while the State Police enter from the other end and arrest them all. The Navy is alerted and intercepts the submarine to find Morgan Todd being held hostage.

Original edition (1929)

• Ghostwriter: Leslie McFarlane
• Summary: A woman asks the boys to track down her missing brother, whom they discover is suffering from amnesia and living as a hermit in the caves near Bayport.

Revised edition (1964)

• Revision type: Completely rewritten; missing-man story retained as a subplot, but new main plot involves sabotage of a radar station.
• Ghostwriter: Andrew E. Svenson