Documentary that examines the use of poison gas as a weapon during World War I. The trench warfare conditions in which soldiers fought in WWI are now recognised as some of the most unpleasant combat conditions of all time. As if the hazards of trench foot, shelling and going 'over the top' weren't bad enough, deadly gases such as chlorine came into being as a means of warfare during the conflict. This documentary traces the history of gas warfare, from the German use of chlorine gas at the Battle of Ypres in April 1915 to the dual Allied response of developing gas masks and their own methods of poison gas delivery.