The frontier between France and Germany runs through three centuries of fortification and war, and this film traces it from Louis XIV's campaigns to the Second World War. Vauban's star forts anchor the earliest chapter, built to lock down a border the Sun King's armies had just seized, and the documentary uses aerial footage of their surviving ramparts to show how geometry became a weapon. From there it moves through the Séré de Rivières network built after 1870, into the concrete galleries of the Maginot Line, and across to Germany's answering Siegfried Line, with underground bunkers and tunnels explored on camera and period maps laid over the modern landscape. The throughline is military engineering rather than battles alone: how each generation of forts tried to solve the failures of the last, and how Hitler's forces eventually went around rather than through. Abandoned casemates and overgrown trench lines carry the history as much as the narration does, turning a strip of the Rhine valley into a timeline of how borders get built, defended, and broken.