August 1914, two weeks after the First World War was declared. 80,000 British troops have arrived in southern Belgium, but they have yet to engage the enemy. Amongst these professional soldiers are the 4th Battalion, The Royal Fusiliers, some of the best trained gunners in the British Army. Lieutenant Maurice Dease and his friend, Australian Lieutenant Fred Steele, command two companies of riflemen and machine gunners who find themselves camping overnight at Nimy Bridge, the northernmost point of the Mons-Conde canal. The canal, surrounding the town of Mons, supports 18 bridges stretched along 20 miles of water - but everyone, including Private Sid Godley and his mate Private William Holbrook, expects to be crossing those bridges in the morning. Overnight, however, intelligence reports the Germans may be closer than the British think, and the Royal Engineers, including Lance Corporal Charles Jarvis and his sapper, Private Neary, are ordered to start preparing the bridges for demolition. What happens next, on Sunday 23 August, tests the soldiers' skill, courage and ingenuity to the absolute limit, leading to the first Victoria Crosses of the First World War. Not everyone will make it through the First Day.

1916, and Britain's armed forces are decimated. Across the country, friends join up to fight together, in new battalions known as 'Pals'. Paddy Kennedy, a warehouse clerk from Manchester, joins the Manchester Pals Battalion alongside his friends Henry, Andy and Jim. They soon discover that the whole of their neighbourhood has joined up, including a nearby factory foreman, now their sergeant, Mitchell and their friend Lizzie, who has become a volunteer nurse. These pals are set to become part of the largest offensive ever mounted by the British Army, the Battle of the Somme. But after a successful first day, they are ordered to seize control of Trones Wood, a thickly forested copse riddled with enemy soldiers and impossible to navigate. Through all this, they survive shellfire and ambush, evade capture by the Germans and encounter the mysterious Private William Hunt, whose fate will become forever entwined with the life of Paddy Kennedy. Paddy's journey leads to the deadliest mission of all - the execution by firing squad of one of his pals. How will Paddy survive becoming a soldier?

1918, and after four years of combat the British invent a new weapon designed to break the stalemate. Private Chas Rowland, determined to bring the war to an end and return home, joins the Tank Corps under the command of Lieutenant Mould, entering the extraordinary world of the Niveleur, a Mark V tank. Primed to take part in a major offensive against the Germans just outside the French town of Amiens, the crew of the Niveleur include Fred Firth, a father-of-two desperate to see his children again; Private Dodds, a tough warrior scarred by the loss of his brothers; and Mike Weston, a civilian mechanic conscripted into the tanks. The crew will only survive four gruelling days behind enemy lines if they can learn to work together. Can they bring the war to an end and make it home?