Plot summary

Innocently responding to a motorist's request that they shut off a light at his home, the Hardy Boys discover a deep mystery: the man used the name of a man, John Mead, that Chief Collig claims died five years earlier in a car accident. Adding to the mystery, the Mead mansion's doors have neither knobs nor visible keyholes. Only after speaking to a locksmith do they learn that the locks were concealed.

Meanwhile, their father Fenton assigns them to investigate a lead in the kidnapping of a doctor that may lead down the trail to a local boy who fell in with a local thief, a master criminal, who's a relation to the boy. The Hardy boys are to locate a traffic signal that hums like someone singing faintly, and drive ten minutes from it in each direction, then investigate the area for a "secret panel".

Fenton's mystery ends up intertwining with the Mead mansion and the master criminal, who's been carrying out a series of break-ins and thefts without triggering the alarm systems. It turns out that the deceased Mr. Mead was an electronics genius who developed a device that could open any lock and defeat alarm systems, but asked that, upon his death, it be turned over to the FBI. The master criminal had befriended Mr. Mead, found out about the device, and stolen it.

Original edition (1946)

• Ghostwriter: Harriet S. Adams
• Summary: The boys' search for a kidnapped doctor leads them to a mysterious mansion whose doors have no visible knobs or locks.

Revised edition (1969)

• Revision type: Minor revisions.
• Ghostwriter: Priscilla Baker-Carr