The Black Stone is the name of an anarchist group which has pledged to carry out the assassination of the Greek Prime Minister. Richard Hannay uncovers the group and its dastardly plot by using the little black notebook left to him by Franklin P. Scudder, the American spy. By deciphering Scudder's code, Hannay is able to establish that The Black Stone wants to start a European war, steal British naval secrets, and blockade the United Kingdom.
Although Hannay is unable to prevent the assassination of the Greek Prime Minister, he is able to track down the members of The Black Stone using the clues in Scudder’s notebook. He finds that they’re a group of German spies playing tennis by an English villa on the Kent coast. In the ensuing struggle, Hannay manages to apprehend two of the spies; the third is subsequently captured by the British authorities.