It is based on upon the memoir Mémoires d'un agent secret de la France libre et La Ligne de démarcation by Gilbert Renault under his pseudonym Colonel Rémy. A small village in the Jura is split by the river Loue which creates the line of demarcation between Nazi occupied France and freedom. A French officer, Pierre, is released by the Nazi soldiers to find his chateau converted into a German command centre. Whilst he is obliged to co-operate with the enemy, his wife Mary supports the resistance movement and is willing to risk her life for it. The Nazis step up their activity against the resistance, insisting that any who attempt to cross the line of demarcation will be shot. When his wife is arrested, Pierre decides to switch his allegiance.