"The Thirty-Nine Steps" is a famous adventure novel primarily because of its film adaptations especially Alfred Hitchcock's version. The hero is Richard Hannay, who features in five of Buchan's novels.
Sometime in May 1914, Europe is close to war. Spies are all over. Richard Hannay, an expatriate Scot has just returned to London from Rhodesia, South Africa, to start a new life. One night, a freelance spy, Franklin P. Scudder, calls on him to ask for help. Scudder claims that he has uncovered a German anarchist plot to murder the Greek Premier, Karolides, and to steal the British plans for the outbreak of war. He has also been following a ring of German spies called the "Black Stone."
A few days later, Hannay find Scudder murdered in his flat. He decides to continue Scudder's work but not to report Scudder’s murder else he might be under suspicion. The mysterious phrase "Thirty-nine Steps" first mentioned by Scudder runs through the story and is eventually revealed in the end. Hannay fears that the murderers will come for him next so he dares not ask help from the police who might otherwise suspect him for murder. He feels it a duty to take up Scudder's cause and save Karolides from the assassination. Hannay decides to hide in Scotland. He escapes by bribing the milkman to lend him his uniform as disguise. Carrying Scudder's pocket-book, he catches a train to Scotland.
Arriving at the countryside near Galloway, he lodges in a shepherd's cottage. The next morning he reads in a newspaper that the police are looking for him. He boards a local train and jumps off between stations. He escapes and finds an inn for the night. The innkeeper shelters him as he tells a modified version of his story. There he cracks the substitution cipher used in Scudder's pocket-book. The next day two men arrive at the inn looking for him. The innkeeper sends them away but when they return later, Hannay steals their car and escapes.
Hannay reflects on Scudder's notes especially the mention of enemy group "Black Stone" and the mysterious "Thirty-Nine Steps." Hannay is now being pursued by an airplane, and a policeman in a remote village tries to stop him. As he tries to avoid an oncoming car, Hannay crashes his own, but the other driver offers to take him home. He is Sir Harry, a local politician. When he learns of Hannay's experiences in South Africa, he invites him to address a meeting that afternoon. Hannay's speech impresses Sir Harry. He writes an introductory letter about Hannay to a relation in London's Foreign Office.
Hannay leaves Sir Harry but is spotted by the airplane again. Miraculously, he meets a road mender out on the moor, and swaps places with him. His disguise fools his pursuers. Next, he meets a motorist, whom he recognizes from London. He forces to exchange clothes with him and to drive him off the moor. Hannay manages to stay ahead of his pursuers and hides in a cottage occupied by an old man, who turns out to be one of the enemy. Fortunately, the room in which Hannay is locked is full of bomb-making materials which he uses to break out of the cottage, slightly injuring him.
Hannay retrieves his possessions from the helpful roadmender while recovering from the explosion. He catches a train to meet Sir Harry's relative, Sir Walter, in his Berkshire home. While discussing Scudder's notes, Sir Walter receives a phone call informing him of Karolides's assassination.
Sir Walter lets Hannay in on some military secrets and leaves for home. Feeling a sense of involvement he returns to Sir Walter's home just in time to see a man, whom he recognizes as one of his pursuers in Scotland, leaving the house. Hannay warns Sir Walter that the man must be a spy perhaps about to return to Europe with his stolen information. At that point Hannay realizes that the phrase "thirty-nine steps" might refer to the landing-point in England from which the spy is about to set sail. Hannay and the British military leaders try to decipher the meaning of the mysterious 39 steps.
Hannay's group decides on a coastal town in Kent with the help of a coastguard. There, they find a path down from a cliff having thirty-nine steps. Offshore they spot a yacht. Posing as fishermen, they visit the yacht, finding at least one of the crew appearing to be German. The people playing tennis by a villa appear to be English, but they match Scudder's description of the conspirators, "The Black Stone." After a struggle, two of the men are captured while the third flees to the yacht, which has been seized by the British authorities. The plot is thwarted. Britain is able to keep its military secrets and it enters the First World War.