While Frank and Joe Hardy, sons of the world-famous private detective Fenton Hardy, are sailing with their friends in a couple of boats, a sudden large storm separates the group. While one boat turns back, Chet and Biff continue sailing out to sea and go missing. After the storm, Frank and Joe search for their friends and find some boat wreckage, leading them to fear the worst. But then they receive a ransom note ... for themselves! Baldy Turk and his gang have kidnapped the wrong pair of boys! Frank and Joe are able to trace the gang to Blacksnake Island, but will they be able to save their friends?