After a devastating tragedy, Joe and Frank Hardy move to Bridgepoint to spend the summer with Aunt Trudy; the boys discover their father has taken on a secret investigation and take it upon themselves to start an investigation of their own.

Joe and Frank's father heads overseas to find Mrs. Khan's nephew, Rupert; Frank discovers an old note of his mother's in Bridgeport that mentions the sunken fishing boat; Joe and Biff befriend a man on the beach who asks them to deliver a message.

Joe finds a strange golden idol hidden in his room; later, he gets lucky at the Bridgeport Fair, but he also draws the attention of the Tall Man; Frank discovers some information about an old mining expedition that ended in tragedy.

When the thief's campsite is discovered, Joe is terrified that the toolbox he loaned the man on the beach has been found by the police; Joe enlists Biff to help him get his monogrammed soldering iron out of evidence at the police department.

Joe and Frank decode a message that leads them to try and make a drop at the warehouse; when the Tall Man escapes from the hospital, the boys reveal their entire investigation to their friends and get their help to lure the Tall Man to Chet's farm.