The soldier "unknown by name or rank" was the focus for an extraordinary outpouring of emotion when he was buried, two years after the end of the Great War.
Crowds lined the streets of London as the body of the Unknown Soldier completed its extraordinary journey, from the battlefields of France, across the channel by warship, by train from Dover to Victoria and then along Whitehall, past the Cenotaph, to be buried with full state honours in Westminster Abbey.
Given the scale of the casualties and the fact that so many were simply unidentifiable, the idea to commemorate the dead through the remains of one unknown soldier, who represented them all, was more than just pragmatic: as an idea it had a symbolic, almost poetic, resonance.
The government had no plan to bring the bodies of British troops back from French soil, and so the tomb of the Unknown became a kind of national cypher for grief, at once individual and for all: he could be any mother's son, anyone's father or brother.
Other Allied nations followed suit.
Millions paid their respects.
As well as telling the story of how the idea of the British Unknown Solder came about, and his extraordinary journey to Westminster Abbey, the programme considers how the future of the Unknown Soldier as a timeless, abstract memorial is now open to doubt - for the reason that thanks to DNA testing, human remains are no longer unidentifiable.