From Vaughan Williams' first film score, this Prelude underscores the opening credits from the 1941 black and white film "49th Parallel" (also known as "The Invaders"). It is a British war film in which a German submarine enters Canadian territorial waters. After they have landed and their boat has been torpedoed, some German crew members are forced to go inland with the ultimate goal of reaching the United States, which was then still neutral. The film was in some ways meant to convince the U.S. to enter the war by warning them that their northern border was vulnerable to a possible invasion by the Germans.