The novel is set during May and June 1914; war was impending in Europe, Richard Hannay the protagonist and narrator, returns to his new home, a flat in London, after living in Rhodesia, to begin a new life. One night he is buttonholed by a stranger, a well-travelled American, who claims to be in fear for his life. The man appears to know of an anarchist plot to destabilise Europe, beginning with a plan to assassinate the Greek Premier, Constantine Karolides, during his forthcoming visit to London. The man reveals his name to be Franklin P. Scudder, a freelance spy, and remarks that he is dead, which holds Hannay's attention. Scudder explains that he has faked his own death to avert suspicion. Scudder claims to be following a ring of German spies called the Black Stone who are trying to steal British plans for the outbreak of war. Hannay lets Scudder hide in his flat, and sure enough the next day another man is discovered having apparently committed suicide in the same building. A couple of days later Hannay returns home to find Scudder dead with a knife through his heart.

Hannay fears that the murderers will come for him next, but cannot ask the police for help because he is the most likely suspect for the murders as he lived in the same building. He also feels a duty to take up Scudder's cause and save Karolides from the assassination. He decides to go into hiding in Scotland and then to contact the authorities at the last minute. To escape from his flat unseen, he bribes the milkman into lending him his uniform and exits wearing it, escaping from the German spies watching the house. Carrying Scudder's pocket-book, he catches an express train leaving from London's St. Pancras Station. Hannay fixes upon Galloway, in south-west Scotland, as a suitably remote place in which to make his escape and remembers somehow the town of Newton-Stewart, which he names as his destination when he buys his ticket from the guard.