In 2009, in an astonishing discovery, the bodies of 250 British and Australian World War I soldiers were found in unmarked graves near Fromelles in northern France. It's the largest war grave to be found in Western Europe in modern times.

This film tells the story of how, over the course of a year, the bodies have been carefully exhumed and many of them identified via DNA matches with living relatives.

WWI: Finding the Lost Battalions features three British families who hope to discover whether their relatives are amongst the dead, and lay to rest family mysteries that have lasted almost a century.

Their relatives are men who went missing in action, but no confirmation of death could ever be given by the War Office. Two are from the sleepy Buckinghamshire village of The Lee, which gave most of its young men to the 2nd Bucks Battalion, whose story this film follows in particular detail.

Drawing on personal possessions found with the bodies, including a bible with handwritten annotations, a heart-shaped leather pouch and a return train ticket, as well as astonishing personal diaries and letters from the men who went missing, the film brings the horrifying truth of the Battle of Fromelles, and its impact on the subsequent generations, vividly to life.

The fighting took place at the same time as the Battle of the Somme, at the height of WWI. More than 1,500 British and 5,533 Australian soldiers were killed, wounded or taken prisoner during 12 hours of carnage between 19 and 20 July 1916. The Allies failed to gain an inch of ground.

Channel 4 was granted exclusive access by the Commonwealth War Graves Commission to document the exhumation of the soldiers' remains and their subsequent burial in the new Commonwealth War Graves Cemetery - the first to be built in 50 years.