As the First World War rages, British intelligence has uncovered rumors of a German plot to sow chaos on the Eastern Front by inciting a holy war among the Muslims of the Near East. With only three obscure words left on a scrap of paper by a deceased spy to guide him on his mission, Major Richard Hannay must make his way to Constantinople and discover the secret of Greenmantle before the Germans can initiate their plan and upend the global conflict.