Fifteen-year-old Pete Jenkins meets two boys and a girl near his summer cabin in northern Michigan, The new friends find clues linking a hermit writer to a counterfeit money scheme. Setting aside other activities for the intrigue of spying, they follow the elderly man by sail to nearby Mackinac Island. To their horror, the tale turns to one of murder. A test of wills leads them through a series of terrifying obstacles to a remarkable conclusion.