Answer: The 39 Steps
It was based on a 1915 book by John Buchan, who featured the hero of "The 39 Steps", Richard Hannay, in four subsequent novels. In this first story, Hannay must investigate a spy ring while on the run for a crime he did not commit. The movie was remade in 1959 and 1978, but chances are if you catch it on PBS, or a cable network other than BBC, you'll be seeing the original classic.