The episode mentions increased troop movements down to the south coast and that "the end of the war is in sight", indicating a pre-D Day setting. The cartography activity at fictitious Beverly Lodge (filmed at Langley Park, Slough, Berkshire) is based on the secret map-making activities undertaken at Hughenden Manor during World War II, which was not known until two years before the shooting of this episode. Anthony Horowitz based much of the story on the experiences of Victor Gregory, a cartographer at Hughenden, and who was engaged as a consultant during the shooting of the episode. Another theme is various efforts by the Church to: preach forgiveness of the enemy; establish relations with the German church (such as the German Confessing Church); and, grant Germany a conditional (rather than unconditional) surrender to prevent the unnecessary killing of innocents by indiscriminate bombing of German cities. The efforts of Dietrich Bonhoeffer are mentioned, as are events reflecting the real-life George Bell, Bishop of Chichester.