In December 1915 the Germans deploy a deadly new form of gas - phosgene, whose horrific symptoms only manifest 48 hours after exposure. Ill informed soldiers continue to fight unaware they have been exposed.

The British 6th Division are on their knees, but one young and principled soldier decides the incessant howling of a wounded dog on the battlefield will cripple the remaining troops' morale. He ventures out into no-mans land, only to find his beliefs challenged in a way he could never have expected.