A dying criminal confesses that his loot has been secreted "in the tower." Both towers of the looted mansion are searched in vain. It remains for the Hardy boys to make an astonishing discovery that clears up the mystery.
When Frank and Joe try to help their father locate Miss Todd's missing brother, they find themselves tangled up with a mysterious hermit named Captain Royal.
Frank and Joe unravel two mysteries, one involving a counterfeiting case and the other concerned with a national security case their father is working on.
The Hardy boys find themselves in a winter adventure on a rock-bound island.
When Frank and Joe try to get hold of a top secret invention, they are put on the trail of jewel thieves and smugglers.
Frank and Joe solve the mystery of the secret locked room in the spooky Dalrymple Mansion.
Falsely accused of a robbery at the airport, the Hardy boys turn the tables on the real thieves by nabbing them in the act of committing another robbery.
The Hardy boys track down the saboteurs who kidnapped their father.
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