The events seen in this movie are based on a true story of an attack by Italian "human torpedoes", which successfully blew up the "Queen Elizabeth" and the "Valiant" on December 18, 1941. The real Bianchi was not shot, but lost his grip on the torpedo when it dove, making it impossible for his commander de la Penne on his own to attach the machine physically to the ship's hull as they had intended. (He later explained that his refusal to talk had been because the British could have saved their ship very simply if they had realized the true nature of the "mine", which was simply lying on the bottom of the harbor) The two men were imprisoned in the bottom of the warship as depicted here, and finally five minutes before the explosion was due, de la Penne sent a warning to Captain Morgan, enabling him to muster the crew safely on the top deck (although not to abandon ship). Neither of the Italians was injured in the explosion. The British did successfully conceal from Italian espionage the damage to both warships for months.

After the capitulation of Italy, de la Penne went to fight on the British side of the underwater war. When, in March 1945, he was being presented with the Italian equivalent of the VC for his role in the attack on the "Valiant", Captain (by then Admiral) Morgan was present at the ceremony, and he pinned the medal onto the man who had once been his enemy.