Two friends on a yachting cruise notice suspicious coastal defenses and charted shoals, leading them to investigate a secret naval plot. They combine careful navigation, charts, and local reconnaissance to trace enemy movements among islands and fiords, suffer close calls and diplomatic obstacles, and repeatedly risk capture to secure evidence. While official channels react slowly, their ingenuity and seamanship enable them to alert the proper authorities and frustrate the planned operation. The narrative foregrounds practical seamanship, detailed chartwork, and the uneasy intersection of amateur courage and national security.