During the German invasion of Belgium in 1914, Jules Jaspar, a chaplain in the Belgian army, is assigned to protect three mentally ill soldiers deemed unfit for duty. After leading the soldiers through dangerous terrain to mighty Fort de Loncin, the Battle of Liège rages around and above them. Jaspar and "the madmen" struggle to keep their wits in the underground fort as the Germans launch shell after shell without mercy, punishing the Belgian 3rd Division as they refuse to surrender.