Two teenage friends, Stanley Sandborn and John Tallman, set out aboard their sloop for the remote Porpoise Island and discover a strange gray speedboat and a hidden inlet called Black Cove. Their seaside outing quickly becomes a detective adventure as they use cameras, fingerprint kits, and seamanship to investigate suspicious craft and outlaw activity. Encounters escalate into capture, escapes, and armed confrontations that draw in federal agents and force the boys to confront betrayal and danger. The narrative moves from boating exploration to pitched fights and a final revelation about the secret hidden in Black Cove.