A tornado has hit Bayport, but the deadliest storm is still brewing -- in the Hardy home. Years ago, Fenton Hardy sent a criminal named Leonard Mock to prison. Now Mock's son has returned to Bayport to seek revenge. He has vowed to put Frank and Joe's father away for good!

But Leonard Mock lost his son to adoption when the boy was five. His name was changed, and the only clue to his identity is that he has since befriended the Hardy boys. Then the brothers uncover a shocking and horrifying piece of evidence. The person out to murder their father may very well be none other than Chet Morton -- their best and most trusted friend!