Throughout most of World War I, trenches and barbed wire ran across the entire continent of Europe. On July 1st, 1916, after a devastating artillery bombardment lasting more than a week, 100,000 British soldiers waited in their trenches ready to advance on the German lines. They had been told to expect minimal resistance but, as they picked their way slowly across no-man's land, guns opened fire. Shells burst overhead and wave upon wave of men were machine-gunned down. It was a military catastrophe of unprecedented proportions. Filmed on the battlefield itself, in laboratories and on firing ranges, archaeologists, military historians and other experts investigate all the factors that culminated in the bloodiest day in the history of the British Army.