When the Hardy Boys set out to solve the mystery of their missing chums, they discover a gang hide-out in a cave on an island along the way.

In a search for the missing Willard Grafton, Frank Hardy and younger brother Joe, encounter a gang of criminals intent on defrauding the US government, and are lead across California and even into Mexico.

When the daughter of Bayport's amusement park owner goes missing, Joe and Frank follow clues to find the girl and determine if her disappearance is related to that of another missing person.

Brother detectives Frank and Joe return to dangerous Black Bear Mountain to help locate a scientist gone missing from a remote research station.

"Frank and Joe are off to the Florida Keys to work alongside Lookout Shark Lab's Dr. E. Ella Edwards, one of the world's foremost marine biologists. On their first evening, Dr. Edwards paddleboards out to check on a lemon shark nursery. But after a sudden squall rolls in, she doesn't return, and the next day her board washes up with a shark bite taken out of it. The Hardys volunteer to help the search and rescue efforts as panic sparks a rift in Lookout...

During an investigation of missing children in Misty Falls, Idaho, Frank Hardy has disappeared, just after the main suspect is found dead.

While at a comic book convention, the Hardy brothers and Frank's new girlfriend Jones meet up with Jones's online friend Harper, but when she disappears overnight, the brothers start looking into her life and uncovering her secrets.

When the Hardy Boys join their father on a trip to Arizona's magnificent Organ Pipe Cactus National Monument, the trip becomes dangerous as they set out to find the black marketeers who are stealing rare cacti.