When Richard Hannay, the hero of "The Thirty-nine Steps", is recalled by the Head of British Intelligence from the Western Front at a critical moment in the battle for France, he has little idea that his contribution to the war effort will be much more crucial than the command of his Brigade in Flanders.

In his strange odyssey to unravel the most sinister of conspiracies - to defeat the allies in the West - he travels from an idyllic manor house in the Cotswolds to a provincial Garden City where pacifism is the order of the day, through Scotland and London under attack, and thence back to the trenches, and the greatest battle of the First World War. There, amid the devastation and the squalor, he finds both love and a horrifying glimpse of chemical warfare before the thrilling denoument in the skies above the battlefield.