It's the start of August 1916, and war is raging on the battlefields of France. In the front-line trenches, British soldiers are vanishing without trace. Captain Mark Steadman witnesses one of his comrades being pulled down beneath the mud and realises that something is terribly wrong. When three civilians and a large blue box arrive on the scene, Steadman finds himself helping in their enquiries. The Doctor declares that something has gone wrong with the timeline of the war; the Battle of the Somme should have already begun, yet there is no sign of it. He demands an audience with Field Marshall Haig! A crater in no-man's land reveals the grisly horror of the missing men's fate and brings the Doctor and his friends into terrible danger. There are forces at work here which even he can't fully comprehend.