"Put her into Brixton jail with all the other 18Bs", says an annoyed lady diner on seeing Sally. This references Regulation 18B of the Defence (General) Regulations 1939, which allowed for those suspected of being Nazi sympathizers to be interned; some indeed being housed in H.M.P. Brixton.

Cyril Fletcher recited one of his Odd Odes in the cabaret (as he genuinely did during World War II), ending with the knowing comment: "Was her journey really necessary?" As well as being a sly dig at Sally Maitland (Anna Neagle), this alludes to the contemporary posters produced by the Railway Executive Committee which urged the public not to make unnecessary use of the railways.

This movie appears to amalgamate various real events of September 1940. "The Evening News" headlines a time bomb explosion outside Buckingham Palace. The Palace was bombed seven times during the war (including an incident with a fifty kilogram delayed action high explosive, which harmlessly landed in the grounds), most notably on September 13, when a bomb landed in front of the Palace, damaging Queen Victoria's memorial and razing the chapel. However, King George VI and Queen Elizabeth were in residence during this morning attack, unlike in this movie.

Cyril Fletcher received an "introducing" credit.

When Jan Orlock (Albert Lieven) explains the German plot to Sally Maitland (Anna Neagle), he says that the plot was inspired by a World War I-era explosion in the Port of Halifax. In fact, he pretty accurately describes the events that took place on the morning of December 6, 1917. Two vessels - the Norwegian S.S. Imo and the French S.S. Mont-Blanc - collided in the port at very low speed (about one knot). The Mont-Blanc was loaded with explosives, benzol (a highly flammable fuel), and other volatile cargo. After the two vessels struck each other, drums of benzol on deck broke loose and flooded the deck. As the two ships disengaged from each other, sparks ignited the fuel, started a fire, and caused an explosion approximately twenty minutes after the initial collision. The explosion killed nearly two thousand people, levelled all structures within a half-mile, and caused a local tsunami.

The Shakespearean scholar at the beginning of this movie quotes: "Be not afeard; the isle is full of noises,/Sounds and sweet airs, that give delight and hurt not./Sometimes a thousand twangling instruments/Will hum about mine ears..." The lines are spoken by Caliban in "The Tempest" (III.2.136).

According to the AFI Catalogue entry for this movie, the U.K. release has a running time of one hour and thirty-five minutes, the U.S. release was cut to one hour and twenty-four minutes.

All location sequences showing Halifax were "B" roll, or second unit shots.

This movie received its earliest documented U.S. telecasts in Los Angeles, California on Tuesday, March 27, 1956 on KHJ (Channel 9), and in Memphis, Tennessee on Wednesday, April 4, 1956 on WHBQ (Channel 13).

The scene of the morning following the boarding of the ship by the crew of the raider, Mrs. Towcester - played by Margaret Rutherford - states that "From now on I am going to live! I am moving to Balham.". Margaret Rutherford was born in Balham.

When Lieutenant Commander Jim Garrick (Richard Greene) shoots at Madame Orlock (Lucie Mannheim), he says, "Not you mama, sit down." This was a catchphrase from the radio show, "Hi Gang", which was very popular at that time.