Along the French Atlantic coast, Hitler's navy poured concrete into five massive bunkers designed to shelter and rearm U-boats between raids on British convoys, structures so thick that Allied bombs barely scratched them. Historians Luc and Marc Braeuer work through newly available German national archive documents to reconstruct how these bases at Lorient, Saint-Nazaire, Brest, La Pallice and Bordeaux functioned, from torpedo storage to crew quarters built to keep the fleet running through the Battle of the Atlantic. The film pairs this with the Allied side of the fight: codebreakers cracking naval Enigma traffic, spies feeding intelligence on U-boat movements, and the shift in tactics, radar, sonar, escort carriers, that eventually turned the hunters into the hunted. Archival photographs, wartime footage and on-location shots of the surviving bunkers anchor the narration, which treats the bases less as ruins than as evidence of a strategy that nearly worked. The result is a detailed account of one campaign that, by most historians' reckoning, decided the war's outcome in the Atlantic.