The year is 1914, and Richard Hannay (Robert Powell), a mining engineer who is visiting Britain for a short time before returning to South Africa, is shocked when one of his neighbors, Colonel Scudder (Sir John Mills), bursts into his room one night and tells him a story that Prussian sleeper Agents are planning to start World War I by murdering a visiting foreign Minister. However, Scudder is murdered, and Hannay is framed for the death by the sleepers. Fleeing to Scotland, Hannay attempts to clear his name and to stop the agents, with the aid of Alex Mackenzie (Karen Dotrice), but not only is he is chased by Chief Superintendent Lomas (Eric Porter) for Scudder's death, but by the agents, who are headed by Appleton (David Warner), who has managed to hide himself in a high-placed position in the British Government.