I have read a couple of reviews which suggest that this episode - the last one filmed - should be swapped with the previous episode. I am going to assume that part of the reason why is because Ms Troy is not in this story. As Alleyn's love interest, she adds spark and substance to all the plots and episodes she's in. She softens his nature and makes him more approachable. Yet, Ngaio Marsh did not write Troy in all the plots, even after their marriage. In some, she's only mentioned when Alleyn writes her a letter. Perhaps the main reason why this episode is judged as an unsatisfactory final episode is, simply, because it was not planned to be the final episode. Perhaps more were planned but management decided against the expense - period dramas are much more expensive. Luck of the draw, or just bad luck.
Review of Scales of Justice
