Greenmantle
John Buchan, 1st Baron Tweedsmuir
 The book opens in November 1915, with Hannay and his friend Sandy convalescing from wounds received at the Battle of Loos. Hannay is summoned to the Foreign Office by Sir Walter Bullivant, a senior intelligence man, who Hannay met and assisted in The Thirty-Nine Steps. Bullivant gives Hannay an outline of the political situation in the Middle East, and hints that the Germans and their Turkish allies are plotting to cause a great uprising throughout the Muslim world, that will throw the whole of the Middle East, India and North Africa into turmoil; Bullivant proposes that Hannay takes on the task of investigating rumours. The only clue he is given is a slip of paper left by a spy, Bullivant's own son, recently killed in the region, bearing the words Kasredin, cancer and v.I. Despite his misgivings and feelings of inadequacy for the task, Hannay accepts the challenge, and picks Sandy to help him. Bullivant tells him that an American, John Blenkiron, will also be useful to him. The three meet up, ponder their clues, and plan to head to Constantinople. They start on November 17, and plan to meet up in a rough hostelry there exactly two months later, going each by his own route - Blenkiron, as a neutral, travelling through Germany as an observer, Sandy using his contacts in the Arab world to make the journey through Asia Minor, and Hannay, taking on the identity of Boer "Cornelis Brandt", entering enemy territory via Lisbon. Arriving there, he meets by chance his old comrade from adventurous times in Africa, Boer Peter Pienaar, and together the two enter Germany via the Netherlands, posing as anti-British exiles itching to fight for the German side. They meet the powerful and sinister Colonel Ulric von Stumm, and persuade him they can help stir up the Muslim peoples to join the German side. The two are separated, and Hannay is introduced to a Herr Gaudian, famed mining engineer (who would later reappear after the war in The Three Hostages), hears of the mysterious Hilda von Einem, and has a brief meeting with the Kaiser. Finding Stumm plans to send him to Egypt via London, Hannay flees into the snowbound countryside, tracked by the vengeful Colonel. He falls ill with malaria and is sheltered over Christmas by a poor woman in a lonely cottage. On his sickbed, he realises that the clue "v.I" on the piece of paper may refer to the name he overheard, von Einem. Recuperated, he carries on, travelling by barge carrying armaments down the Danube, picking up with Peter Pienaar, who has escaped from a German prison, along the way. They pass through Vienna, Budapest and Belgrade, and as they travel, Hannay connects the phrase "der grüne Mantel" with something else he overheard earlier. They reach Rustchuk on January 10, with a week to go before the rendezvous in Constantinople. On arrival there, Hannay has a run-in with Rasta Bey, an important Young Turk, and intercepts a telegram showing his trail has been picked up. They carry on by train, fending off an attempt to stop them by the angry Rasta Bey, and reach Constantinople with half a day to spare. They seek out the meeting place, and are attacked by Bey and an angry mob, but rescued by a band of mysterious, wild dancing men, who they then antagonise. Next day they return to the rendezvous, an illicit dance-room, where they find the main entertainment is none other than the wild men of the previous day. At the climax of the performance, Enver's soldiers arrive and drag Hannay and Peter away, apparently to prison, but they are instead delivered to a cosy room containing Blenkiron and the leader of the dancers - none other than the miraculous Sandy Arbuthnot. They pool their news - Sandy has identified Kasredin from the their clue sheet, as the title of an ancient Turkish allegorical story, the hero of which is a religious leader called Greenmantle, and has also heard much of a prophet known as "the Emerald", associated with the play. Blenkiron has met and been impressed by Hilda von Einem, who is in Constantinople and owns the house in which they are staying. Blenkiron provides Hannay with a new identity, an American engineer named Hannau, and they attend a dinner party where they meet Herr Gaudian again, and Enver himself. Lost out riding, Hannay encounters von Einem, and is fascinated by her; later, he is recognised by Rasta Bey, and has just knocked him out and hidden him in a cupboard when von Einem arrives. Hannay impresses her, and hears she plans to take him East with her. Sandy visits, agrees to deal with the captive Turk and provides news of his own - the clue Cancer means the prophet Greenmantle has the disease and is on his deathbed. Blenkiron joins them, and tells them that fighting has hotted up between the Russians and the Turks, and they deduce that they will be taken towards Erzerum to help with its defence. On the long road to Erzerum, they crash their car, and spend the night in a barn, where Hannay has a vivid dream of a hill with a saucepan-like indent in the top. They carry on on worn-out horses, but seeing a new car by the roadside, they steal it, only to find it belongs to Rasta Bey. They make good speed onward, but on arrival in Erzerum, they are delivered straight to Stumm, who recognises Hannay and has them locked up. They are rescued by one of Sandy's men, steal some plans from Stumm, and escape across the rooftops. With the battle of Erzurum booming in the background, they realise the importance of the stolen plans, and Peter Pienaar volunteers to sneak through the battle lines and deliver them to the Russians. Sandy appears, magnificently dressed, and reveals that Greenmantle is dead and that he himself has been chosen to impersonate him. They form a plan to flee around the side of the battle lines, and while Sandy's helper searches for horses, Pienaar sets off on his dangerous mission. Pienaar has an eventful and terrifying journey across the battlefield, while Hannay and Blenkiron hide out in a cellar. On the third day, they break out, and make for safety in a wild horse ride, closely pursued by their enemies. On the verge of capture, they find the hill of Hannay's dream, and entrench there, holding the enemy at bay. Hilda von Einem comes in, and appeals to them to give up, but they refuse; she is shocked to learn Sandy is a British officer, and as she leaves, she is slain by a stray Russian shell. Stumm arrives with artillery, and their position looks sure to be destroyed and overrun, but Stumm waits till dawn to savour his revenge. Just in time, the Russians, helped by the plans delivered by Pienaar, break through the defences and sweep towards the town. Stumm's men flee, Stumm is killed, and Hannay and Sandy meet up with Pienaar to ride into the city and victory.