Operation Source, the real life mission on which this film was based, was actually considered a failure in many respects. The Germans easily repaired the damage to Tirpitz. Historians William Garzke and Robert Dulin remarked that the successful repair effort was "one of the most notable feats of naval engineering during the Second World War."
In the scene where Lieutenant Tom Corbett (Donald Sinden) and his crew first board the midget submarine, the view of three Castle-class frigates tied alongside each other can be clearly seen in the background, this view of the three frigates was also seen in "The Cruel Sea (1953)," when Ericson and Lockhart first take charge of the factitious H.M.S. Saltash Castle after they survive the sinking of their first ship H.M.S. Compass Rose.
This movie was the sixth most popular movie at the British box-office in 1955.
The success of this movie caused Sir John Mills to be voted the most popular movie star in the U.K. that year.
When this movie was made in 1954, Queen Elizabeth II was on the throne, but throughout the war, it was her father, King George VI, who was the monarch. Whether by design or chance, most of the cast have historically accurate cap badges, which is to say they include a King's crown. The notable exception is Sir John Mills, whose cap badge has a Queen's crown.