The story's narrator, Richard Hannay, arrives in London from Rhodesia early in 1914, having made a modest fortune as a mining engineer. Disillusioned with his uneventful life as a man about town, he is on the brink of resolving to leave England for good when a panicked neighbour, Franklin Scudder, knocks at the door of his flat in Portland Place. Scudder is a freelance journalist who claims to have uncovered a plot against the Premier of Greece, Constantine Karolides. According to Scudder, Karolides is to be assassinated in London in a few weeks' time, on 15 June, an event which the plotters hope will trigger war in Europe.
Fearing for his life, Scudder has gone to the trouble of faking his own death, and needs to disappear from view. Hannay permits him to hide in his flat, and is horrified when a few days later he returns to find Scudder with a knife through his heart, now truly dead. Determined to warn the government of the plot, but unwilling to go to the police for fear of being arrested for murder, Hannay escapes the building disguised as a milkman and takes a train to Scotland, intending to find a remote area where he can lie low. He takes with him the coded notebook in which Scudder had recorded his findings.
Hannay alights at a rural station in the Galloway Hills, and a cat-and-mouse chase ensues as he evades both the plotters, who attempt to spot him on the open hillside from an aeroplane, and the police. Decyphering Scudder's notes, he learns that his adversaries are members of a German spy ring known as the "Black Stone" whose goal is to steal Britain's naval defence plans before war breaks out. Hannay meets Sir Harry, landowner and local parliamentary candidate, and takes him into his confidence. Sir Harry promises to write to his godfather, Sir Walter Bullivant, Permanent Secretary at the Foreign Office, to warn him of the plot.
Narrowly avoiding his pursuers, Hannay stumbles into a lonely cottage and finds himself face to face with the Black Stone's leader. Hannay's lies are convincing enough to leave the spies in doubt as to his true identity, and they lock him in a storeroom rather than killing him outright. Finding a cabinet full of explosives, Hannay uses his experience as a mining engineer to escape by blowing the window from its frame. Eventually he manages to catch a train south, hoping to find Sir Walter Bullivant at his home in Berkshire.
Sir Walter accepts the bulk of Hannay's story but doubts that Karolides' life is in danger. An urgent government phone call, however, informs him that Karolides is already dead. The two men travel to London, where Sir Walter is to host a high-level official meeting at his city townhouse. Hannay, now cleared of the Portland Place murder, is left to his own devices, but a general feeling of unease prompts him to call at Sir Walter's house. He arrives just in time to see the First Sea Lord leaving; their eyes briefly meet, and Hannay recognizes him as one of the spies in disguise. Hannay breaks into the meeting, but by the time the deception is confirmed the man has long gone, taking with him the naval secrets he has just learned.
Realising that the spies will have to cross the Channel to get their information back to Germany, Hannay and the meeting attendees comb Scudder's notebook for clues as to the planned point of departure. An entry reading "Thirty-nine steps—I counted them—High tide, 10.17 p.m." leads them to a clifftop villa in Kent with a private flight of steps – 39 in total – running down to the sea. A yacht waits offshore. Hannay confronts the occupants of the villa and is mortified to find what appears to be a perfectly ordinary group of English friends who have been enjoying a game of tennis in the sun. But then one of the men droops his eyelids in a characteristic gesture that Hannay recognizes – it is the owner of the cottage in Scotland. Hannay blows his whistle, and the spies are arrested before they can reach the yacht. Britain enters the Great War seven weeks later, and Hannay is commissioned as a captain in the army. He comments "But I had done my best service, I think, before I put on khaki.”[5]
