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."
This movie was the sixth most popular movie at the British box-office in 1955.
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.
The success of this movie caused Sir John Mills to be voted the most popular movie star in the U.K. that year.
The watch on Commander Fraser's (Sir John Mills') wrist is the famous Jaeger-LeCoultre Reverso, a watch with a case that can be turned around so that the front faces downwards, thus protecting the glass from hard knocks. This robust watch is nowadays considered as an ultra-luxury item.