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.

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

Frank Hardy has been located. The victims in the past seemed to have been children only, but since Frank was taken, who knows if anyone is safe? Everyone is now a suspect, even Frank.

"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...

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.