A spy on the run accosts Richard Hannay, a former intelligence agent, in his London flat. The desperate man gives him a coded notebook and then is shot dead by German agents bursting into the room. Hannay battles the intruders and escapes. He returns with a constable, who sizes up the evidence and decides that Hannay himself is the killer.
Hannay is thus plunged into the cloak-and-dagger intrigue that precedes the outbreak of World War I. Pursued by police, but also chased by German agents who want the secret notebook, he makes his way to Scotland, where a German spy ring is executing a plan that will render Britain defenseless against invasion.
Along the way, he is joined by combative suffragette Victoria Sinclair, who at first mistakes him for a liberal politician, but is unfazed to learn that he is an accused murderer and, she thinks, a "delusional maniac" on the subject of German espionage plots.
Dangers and surprises lurk everywhere as Hannay and Victoria try to save Britain in her most perilous hour — none as great as what awaits them at the bottom of the "39 steps."
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Warning: Contains significant plot spoilers
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Sitting in his London club on June 28, 1914, former intelligence officer Richard Hannay is bored and alienated. Recently returned from Africa, he finds England stultifying. That is, until he goes to his flat and is confronted by an agitated, gun-wielding neighbor, Scudder.
Hannay disarms Scudder and learns that he is an English spy on the trail of a German espionage ring. Barely coherent, Scudder gives Hannay a coded notebook to deliver to a man named Captain Kell, but then German agents break in and shoot Scudder dead. Before they can harm Hannay, he makes his escape and returns with a constable, who surmises that Hannay is lying about the intruders and is himself the killer.
Soon on the run from an ever-tightening police dragnet and unable to reach Captain Kell, Hannay, makes his way to Scotland in search of the German spy ring. There he deciphers part of Scudder's code and learns that the Germans plan to start a war: first destroying the British fleet and then invading the country.
While fleeing from bloodhounds and a murderous biplane, Hannay stumbles in front of a car carrying Harry and Victoria Sinclair, a brother and sister who mistake him for a political speaker they are to pick up. They transport him to a political rally, where he gives a rousing speech. As police arrive, he makes his escape with the help of Victoria, who is a feisty suffragette.
The police lose Hannay's trail, but not the Germans, who capture Hannay and Victoria and take them to a secluded castle. There they meet the sinister espionage chief, Professor Fisher, but the professor has to absent himself when Victoria's uncle, Sir George Sinclair, arrives asking if anyone has seen his missing niece.
Imprisoned in the castle dungeon, Hannay and Victoria free themselves with the aid of her hair pin and break into an adjoining storeroom of explosives, which they use to blow a hole in the wall and make their getaway. Finding an inn, they treat their wounds from the explosion, while slowly warming to each other's company.
The next day Hannay and Victoria go to Harry's house, where Harry arranges a meeting with Sir George, a British defense advisor who is privy to the naval plans being sought by the Germans. Hannay recounts his saga and the case appears to be winding up.
But various clues lead Hannay to suspect that Victoria is more deeply involved than it appears. He breaks into the naval planning meeting the next day and finds Victoria with the mysterious Captain Kell. She admits that she has been a British spy all along. Could she also be the mole who betrayed Scudder?
A cigarette box carelessly left on the table convinces Hannay that Sir George, who has already left the meeting, is the traitor; that he has used his photographic memory to memorize the defense plans; and that he is about to deliver the secrets to the Germans. But where is this rendezvous to take place?
The coded notebook provides the answer in invisible ink: "39 steps." Hannay and Victoria race to the castle, where they find 39 steps leading down to a dock where a German U-boat awaits Sir George. In the shootout that follows, the spies surrender. Hannay and Victoria seemed destined for a romantic ending, but a dying German gets off a shot that fells Victoria, who tumbles into the water and disappears...
Four months later, Victoria is very much alive and shows up at St. Pancras Station to bid farewell to Hannay, as he departs for service in the conflict that has just started: World War I.