When two of the Hardy Boys' chums, Chet and Biff, go missing during a motorboat trip down the coast, the brothers Frank and Joe set out to rescue them only to be found captured themselves. Below is from wiki. Plot summary (original edition)[edit] The original story opens with Chet Morton and Biff Hooper preparing for a week-long boating trip in Biff's new motorboat Envoy. As Frank and Joe are instructing Biff on the handling of the craft in Barmet Bay, another boat occupied by three men menaces them, nearly causing Envoy to collide with two sailboats. The Hardy Boys note that the three men were closely watching Chet and Biff, which becomes important later in the story. The next day, an excited Chet and Biff leave for their boating trip on the Envoy. Both friend Tony Prito in his boat Napoli and Frank and Joe in their Sleuth decide to escort their friends to the end of the bay to see them off on their journey. Frank and Joe take Callie Shaw and Iola Morton as passengers. But before any of them reach the ocean, a violent squall strikes the bay and the Sleuth is forced to turn back to port, nearly capsizing. Soon after, Napoli also returns and Tony reports that Envoy did not turn back, and that Biff and Chet decided to press on through the storm. The boat was last seen headed out to the open ocean. After three days with no word from their friends, Frank and Joe organize a search effort to find trace of them. Before they can begin, however, their plans are interrupted by the arrival of their Aunt Gertrude, who has come for an extended visit, and doesn't approve of "letting children go out in boats." With the approval of Mrs. Hardy, they eventually begin searching up the coast, accompanied again by Tony Prito in his Napoli. No villages up the coast have seen any sign of Biff's boat, though the boys do find some anonymous wreckage on a reef which leads them to believe that disaster struck the Envoy. They return home in defeat, and Chet and Biff are presumed dead by the community. Several days later, however, their hopes are restored when Aunt Gertrude opens a letter addressed to her brother Fenton Hardy. It is from Baldy Turk's gang, who claim to have kidnapped Frank and Joe, and are demanding ransom for their return. Frank and Joe, obviously not kidnapped, deduce that gangsters must be holding Chet and Biff, mistakenly believing that they are the Hardy Brothers. At this time, Tony also remembers that Biff once mentioned going down the coast, to Blacksnake Island. After one misadventure where Frank and Joe believe they have spotted Chet on a schooner, the boys approach Blacksnake Island to see the same boat which had menaced them on the bay leaving it. They land on the island and search for signs of Chet and Biff, eventually finding them being held captive in a cave by Baldy Turk's gang. After briefly freeing their friends, all the boys are recaptured, and hear news that Fenton Hardy has also been taken by the gang. They escape again, but Frank and Joe get separated from their friends once more in the darkness. They reach their boat, and steal the gang's boats as well, leaving the criminals stranded on the island with Chet and Biff. The Hardys return with the authorities and round up the gang, finally releasing their friends from captivity. They return home to find that their father is also safe, and had arrested Baldy Turk while Frank and Joe had helped capture the rest of his gang.