Regardless of the identity of Major Martin, Nigel Balchin's script stayed as close to the truth as was convenient, yet the film does fall back on some dramatization. For example, the episode of the Irish spy, O'Reilly, is a complete fabrication. The British Security Service controlled the German spy network in the UK with its Double-Cross System, though this fact was still secret at the time the film was made. Ewen Montagu declared that he was happy with the fictitious incidents which, although they did not happen, might have happened.