When a strange young woman is stabbed to death in his flat, Kenneth More finds himself involved in a mysterious adventure involving espionage and murder. Before her death the girl tells him that she is a secret agent and gives him all the clues she knows about a spy organization seeking to smuggle some important plans out of the country. All he knows is that the top man is somewhere in Scotland and that the tangle is tied up with strange words told him by the victim - 'The 39 Steps.' Suspected of the murder of the girl, More has just 48 hours to find out the secret of the 39 Steps, expose the gang and so clear himself of the murder rap.