In the fourth book, called "Fenn Masterson's Discovery," there was related the details of a long trip the chums took on the Great Lakes. They were aboard a vessel commanded by Captain Wiggs, and almost from the start the boys were involved in a mystery. They were pursued by strange men, when they landed to witness a large grain elevator on fire, and eventually they succeeded in causing the breaking up of a gang of Chinese smugglers, and a band of scoundrels who were secretly taking valuable minerals from a cave, under land owned by a man whom the boys had once befriended. It was not until after some strenuous happenings that these events had come to pass, and, more than once, our friends were in danger. But Fenn Masterson succeeded in getting on the trail of the mystery, through an odd discovery he had made, and, though he was captured by the enemy, he used his eyes and ears to good advantage, so that when his friends came he could lead them to the secret cave.