Mr. Standfast (Unabridged‪)‬

Listed in the BBC's 100 Novels That Shaped Our World, Mr. Standfast is the third of Buchan's thrilling, fast-paced World War I adventure novels, following The Thirty-Nine Steps, and Greenmantle, in which Brigadier-General Richard Hannay is reluctantly plucked from the trenches of the Western Front, to undertake a vital secret mission against a German spy ring operating among pacifist elements in England. 

In pursuit of the spy master, Hannay encounters comrades old and new, and earns the love of a beautiful and intelligent woman - but he discovers that his foe is an old adversary, and even more deadly than he had imagined; and he comes to realize that only the ultimate sacrifice can save the day for England....