Then, influenced by a "hunch" on Rawlins' part, Mr. Pauling and his companions had followed a tramp steamer, of which they were suspicious, to St. Thomas. Although there was no evidence conclusive enough to warrant holding the tramp, suspicion pointed to the fact that the leader of the gang of criminals was somewhere in the vicinity. Owing to mysterious radio messages, the party chartered a schooner and went to the neighboring island of St. John.
Here they met a Dutch naturalist named Van Brunt who was dealing with the "reds." Rawlins, spying on him, was held up and narrowly escaped death at the hands of a man whom he recognized as the master criminal they were seeking. Later, this man was found dead and proved to be a person disguised to impersonate the real leader, while Van Brunt visited the schooner and convinced Mr. Pauling and Mr. Henderson that he was innocent and knew nothing of the "red's" activities.
Becoming friendly with the boys, the Dutch scientist took them on a trip into the bush and while they were in a huge cave, deserted them. Soon afterwards a severe hurricane swept the island, imprisoning the two boys within the cavern by a tree falling across the entrance. In the meantime the other members of the party were compelled to seek refuge from the hurricane in the village on shore and were amazed to see the tramp steamer entering the harbor to escape the storm. As soon as the gale was over a searching party started out to find the missing boys and discovered that Van Brunt's house had been destroyed by lightning.
While they were hunting for the boys, Tom and Frank had been made prisoners by a red-bearded man whom they knew was one of the gang. They had been placed on a submarine where Van Brunt confronted them, admitting he was a member of the "reds" and had purposely betrayed the boys. From the submarine they were taken to a locked cabin on a vessel and later were rescued in a most astounding manner by Sam, the Bahaman, who also killed Van Brunt. During their imprisonment the boys had overheard a plot to capture the other members of the party by means of a decoy letter and reaching their friends safely Tom and Frank related their tale in time to save the others from falling into the scoundrels' trap. Soon afterwards a destroyer, which was in constant touch with the schooner by radio, arrived in response to Mr. Pauling's summons. The tramp, in a last desperate attempt to escape, tried to run down the schooner but failed owing to Rawlins' quick wit. Then, turning, the tramp endeavored to leave the harbor by a narrow entrance, but was sunk by a shot from the destroyer's guns.