Plot summary (original edition)

When Raymond Dalrymple starts to receive death threats he seeks help from Fenton Hardy, however Mr. Hardy is out of town so Frank and Joe offer to help. Mr. Dalrymple did not want to give the Hardy boys the case, instead he simply told them to investigate the old Purdy house out on the Shore Road. While walking on the shore road they are almost hit by a big touring car with the curtains down, they then meet a motorcycle cop who was chasing the car which supposedly carried river thieves.

The next day Mr. Dalrymple returned, having decided to let the Hardy boys take the case. He explains that he recently purchased the Purdy house and had been using a secret safe room there to find solitude, however he had received death threats delivered to the secret room while it was securely locked. The Hardy boys explore the Purdy house that night and see a man who looks like Mr. Dalrymple enter the house before they hear screams coming from the house and meet Hurd Applegate, who the boys met while solving The Tower Treasure mystery, running from the house.

Mr. Applegate is too excited to explain what he was doing there, so the Hardy boys have to go back and explore the home another day. Upon further exploration they find an unused wing of the home which has recently had locks added to the doors. The police are called who find the locked rooms filled with stolen goods, stolen by the river thieves. While the police thought this resolved the case, the Hardy boys kept an eye on the Purdy house and found a stranger coming to the house, who then runs away after another scream is heard.

While investigating the house again the Hardy boys are taken hostage by a crazy old man named Amos and another man who looks almost exactly like Mr. Dalrymple. Surprisingly they are rescued by Mr. Applegate and a bomb, which was attached to the ticking grandfather clock, is safely disarmed. Mr. Dalrymple arrives with the police and Amos falls to his death while trying to escape. The Hardy boys examine the device he was using and solve the mystery of how he managed to deliver the death threats to the secret room, as well they locate stamps which Mr. Dalrymple's doppelganger had stolen from Mr. Applegate.

Plot summary (revised edition)

A banker named Ray Dalrymple of Lakeside gets death threats and shows up at the Hardy's house seeking Fenton Hardy's assistance. Frank & Joe offer to help, but at first he's reluctant. At last, he agrees. Soon after that, during a hike with their friends, the Hardy boys are nearly run over by a car, then investigate the Purdy place. After that, they rest and talk over their recent mis-adventures. After this, another stormy sleuth of the mansion reveals a man resembling Dalrymple enter the house. Suddenly, a light goes on in the house followed by a man's scream. This later proved to be an elderly man named Hurd Applegate, a friend of the Hardy boys. He tells them of the theft of his valuable jade collection. Further sleuthing reveals that the thefts are performed in well known speedboats such as the Hardy's Sleuth. Shortly after, Dalrymple disappears and the Hardy's, fearing for his life, go to the Purdy place to look for him. Then the Hardy are held up by a man named Arthur Jensen looking similar to Dalrymple living in his house secretly in hiding. Jensen was a thief who stole small but valuable things from ships, etc. Frank and Joe are bound and gagged along with an inventor named Amos Wandy with a time bomb set to go off at 3 am. When about one minute remains a window panel breaks and Chet Morton emerges and unbinds Frank, Joe and Wandy. They then defuse the bomb just seconds before it was supposed to go off. Then Jensen is later caught on the Purdy mansion grounds and the valuables are retrieved. Another discovery proves that the death threats to Ray Dalyrmple are lowered down through the chimney in the jaws of one of Amos Wandy's invention, which goes by wheel to the middle of the hidden room, then drops the note and is raised up by wire. Then Ray throws a party and gives Mr. Wandy permission to stay in the hidden room and invent whenever he wants.

Original edition (1932)

• Ghostwriter: Leslie McFarlane
• Summary: The boys discover how and why Raymond Dalrymple is receiving threatening notes in a secret room of the old Purdy mansion.

Revised edition (1962)

• Revision type: Completely rewritten; most plot elements retained, but a different villain is added.
• Ghostwriter: James Buechler