Following the murder of two cabinet ministers, in whom he had confided his fears, Colonel Scudder (John Mills), from British Intelligence, takes refuge in the apartment of Richard Hannay (Robert Powell). Scudder has unearthed a plot to assassinate the Greek Prime Minister on a visit to London. Meanwhile the Prussian agents responsible for the previous killings locate Scudder. He escapes in search of Hannay, who is making his way to St Pancras for a trip to Scotland, but is stabbed. Dashing to his aid, Hannay is arrested for murder by the Chief Superintendent Lomas (Eric Porter). His detention is short-lived, and following a chain of events, including being dramatically snatched by the agents and going on the run, he makes the acquaintance of David Hamilton (Miles Anderson) and Alexandra Mackenzie (Karen Dotrice) and falls under suspicion for another murder. Eventually after fleeing from them and escaping once more from the agents he meets up with David and Alexandra again. After finally convincing them of his predicament and innocence they agree to help him unravel the mystery. Alexandra and Hannay go to Strathallan and, as anticipated, find Scudder's real notebook Upon their return to the house, they discover Hamilton murdered. As the distraught couple leave, a shot is fired and Hannay is hit. Hannay, wounded but still very much alive, is now being given every assistance by the police on recognition of his innocence. Scudder's notes are in a complicated code. But suddenly the riddle of Scudder's notebook and "The Thirty Nine Steps" becomes clearer ... but are they too late?