Explosions are raging in one of the ports in the territory occupied by the German fascist invaders. An old and experienced military leader, Admiral Reinhardt, arrives to investigate these emergencies. Together with the head of the Gestapo, SS Standartenfuehrer Hübe, he takes all measures to suppress the activity of the partisans, raids and arrests, but all in vain. Underwater sabotage continues. A detachment of fighters with experience of submarine warfare is sent from Italy to fight the demolitions.