1916: A young woman is found murdered in a cinema. Harvey Marmion and Joe Keedy set out to find the killer who so elusively fled in the dark. Before long, two more victims are found dead. Meanwhile, Marmion's son Paul prepares for life on the front line as he marches to the Battle of the Somme. Suffering a vicious blow in No Man's Land, Paul is left blinded for the rest of his life. Marmion must come to terms with the permanent darkness of his son's life, while continuing to search for the brutal killer who only strikes in the dark.