Beneath the fields around La Boisselle, a small village in Picardy, lies a network of tunnels British troops dug between 1914 and 1916, extending over several kilometres and rediscovered only recently after nearly a century sealed shut. Historian Peter Barton leads a team of archaeologists into these galleries, the first people to enter them since the war, and finds a battlefield that never saw daylight: chalk-cut passages, listening posts, and the remains of some of the more than 120 men who died here. The film reconstructs the underground war waged by tunnelling companies made up of former sewer workers and civilian miners, men who dug toward enemy lines while straining to hear German counter-diggers doing the same, knowing that losing the contest meant being buried alive or blown apart. Barton traces the mines' role in launching the first day of the Battle of the Somme and the missed opportunity that followed, when British commanders failed to exploit the tactical edge those explosions handed them. The tunnels stand as both weapon and tomb.