The Germans built several submarine bases on the west coast of France after their occupation of the country in 1940. Enormous submarine pens of reinforced concrete were erected at Bordeaux, Brest, La Pallice, Lorient and Saint-Nazaire. These structures protected the U-Boats from attack and allowed them freer access to the Atlantic Ocean, as the submarines did not have to transverse the English Channel or go around the UK via the North Sea from bases in Germany. As of 2020 these massive constructions are still largely intact and some have even been re-purposed. The pens at La Pallice can be seen in Das Boot (1981) and Raiders of the Lost Ark (1981).
This film received its initial television broadcast in Los Angeles Friday 3 May 1957 on KTTV (Channel 11), followed by Seattle 8 May 1957 on KING (Channel 5), by Miami 29 May 1957 on WCKT (Channel 7), by Chicago 2 June 1957 on WBBM (Channel 2), by Portland OR 8 June 1957 on KGW (Channel 8), by Philadelphia 24 June 1957 on WFIL (Channel 6), by Amarillo 28 June 1957 on KFDA (Channel 19), by New Haven CT 1 July 1957 on WNHC (Channel 8), by Altoona PA 15 July 1957 on WFBG (Channel 10), by Minneapolis 1 October 1957 on KMGM (Channel 9), by San Francisco 27 January 1958 on KGO (Channel 7) and by New York City 31 January 1958 on WCBS (Channel 2).
The novel by Helen MacInnes upon which this film is based was serialized in The Saturday Evening Post from May 2 to June 20, 1942.