Germany's U-boats turn the North Sea into a killing zone during the First World War, sinking merchant ships without warning and pushing Britain toward a counter-blockade meant to starve the Central Powers of food and raw materials. The film tracks this naval war as an intelligence war as much as a shooting one, following the codebreakers reading German signals and the diplomats measuring how far unrestricted submarine warfare can go before it drags neutral countries into the fight. The turning point is the Zimmermann Telegram, intercepted and decoded by British intelligence, in which Germany proposes that Mexico attack the United States in exchange for lost territory. Its exposure pushes American public opinion and then American troops into the Allied camp. Archival footage and expert testimony trace how a war fought largely underwater and in cipher rooms ends up deciding the balance of the entire conflict on land.