John Buchan 
The 39 Steps 

Europe is close to war and spies are everywhere. Richard Hannay the protagonist and narrator, an expatriate Scot, returns to his new home, a flat in London, after a long stay in Rhodesia, in order 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, Karolides, during his forthcoming visit to London.