Unrestricted submarine warfare by the Germans and British response: various shots of battle at sea involving U-boats, torpedo wakes, defenceless, neutral freighters exploding and sinking. Staged shipboard shots showing passengers panicking, various shots of ships sinking, one on fire. HAS of squadron of British destroyers ("sea dogs") at sea, trailing smoke from their funnels and signalling with blinker lights. PAN of camouflaged merchant ships in convoy. Ship to air shot of sea plane with floats flying past. LS of destroyer cutting through angry sea, CU of torpedo launching tubes. Shots from stern of destroyer of depth charge exploding. Dim LS of oil slick on the ocean's surface, indicating that a sub has been hit. MLS of "mystery ship" (old, bedraggled vessels manned by Royal Navy men used to lure the U-boats into position) approaching. Various shots depicting the attack of a "mystery ship" by a U-boat on rough seas. CU of British Navy's White Ensign flying in the wind and gunners concealed on "mystery ship" commencing their attack. Various shots showing battle between Q-ship and U-boat ending with LS of U-boat being hit and sinking.