A warship returning from a training exercise was tasked with apprehending an unknown schooner that had violated GDR territorial waters. Although nothing illegal was found during the search of the ship, the ship's commander Fischer was suspicious of the owner of the schooner, Arendt, a former lieutenant of a Nazi coastal battery. Fischer's doubts were confirmed when a diver came aboard the schooner and photographed a military object underwater. The investigation showed that the detained schooner was on a sabotage mission, and the "activities" of Arendt, a former Gestapo Obersturmführer, were also revealed.