During the later years of the First World War Brigadier-General Hannay is recalled from active service on the Western Front to undertake a secret mission hunting for a dangerous German agent at large in Britain.

Dick Hannay, under forty and already a successful Brigadier-General with good prospects of advancement, is called out of uniform by his old comrade, spymaster Sir Walter Bullivant, and sent to Fosse Manor in the Cotswolds to receive further instructions.

He is upset by the idea of such a pose, but comforted by thoughts of his friend Peter Pienaar, briefly a successful airman and now a prisoner in Germany, and by the beauty of the Cotswold countryside.

At Fosse, he meets two middle-aged spinsters, their cousin Launcelot Wake, a conscientious objector, and their niece Mary Lamington, a girl whose prettiness had struck Hannay earlier, while visiting a shell-shocked friend in the hospital where she works.

Blenkiron reveals that he has been hard at work for some time, around the world and undercover around England, on the track of a huge network of German spies and agents, with their head somewhere in Britain, leaking vital information to the enemy.

Hannay stays there overnight, and next morning fetches his provisions and searches the beach, finding the deep water of the bay ideal for submarines.

He hides out in a troop train heading south; he gets off when it stops, is seen by Linklater, but at the station hooks up with his old pal Archie Roylance, a pilot who flies him on southwards.

He arrives in London in the midst of an air raid; in a tube station he sees Ivery, the spymaster's guard down in fear, and Hannay finally recognises him as one of the "Black Stone" men he had tangled with in The Thirty-Nine Steps.

He finds Geordie Hamilton, and employs him as his batman; he runs into Launcelot Wake, working as a support labourer behind the lines; he sees several adverts in English and German newspapers, which he suspects may be some kind of coded communication.

Flying with Archie Roylance on a reconnoitre, they get lost in fog and land near the chateau in question, where Hannay sees a mysterious old woman in a gas mask.

Finding the castle is in a vital strategic spot, he returns to investigate, and learns the place is leased by a man named Bommaerts, one of the words he had overheard on the beach in Skye.

Blenkiron gives dinner for Hannay and Mary, now engaged, where he learns that the newspaper advert scam has been broken up and its operatives, led by Gresson, arrested.

The two catch up, swap stories, and await instructions, keeping an eye on the nearby Pink Chalet, believed to be the base of the Wild Birds.

He resumes his command, and holds a thin line against the German advance, with Wake running messages for him, Blenkiron engineering the reserve trenches and Mary nursing in a nearby hospital.

A long and hard battle ensues, in the course of which Wake dies heroically, von Schwabing runs into No Man's Land and is shot by his countrymen, and Blenkiron joins the fray with a party of Americans.