This third film version of The Thirty-Nine Steps stays much closer to the plot of John Buchan's novel than previous adaptations and moves the story's setting back to the eve of The First World War. Colonel Scudder (John Mills) is a British spy who has uncovered a plot to assassinate the Greek premier on his visit to London, something that will spark a crisis in the Balkans and likely lead to war in Europe. When he finds himself pursued by enemy agents determined to kill him and take possession of his evidence against them, Scudder seeks sanctuary in the apartment of a neighbour in his building, Richard Hannay (Robert Powell).