In 1914, Germany spies are everywhere in London. After a spate of assassinations of important British politicians, a retired British intelligence officer, Colonel Scudder, realises his life and his mysterious black notebook are in danger. He turns to Richard Hannay, a mining engineer who is visiting Britain for a short time before returning to Union of South Africa, who happens to be staying in a flat in the same building. Scudder tells Hannay of a plot by Prussian 'Sleeper agent' agents, who are planning to pre-empt a World War I against the Triple Entente powers by assassinating the Greek prime minister visiting the UK.