In all the episodes of this series, you will find no more inspired performances than Max Adrian as the French Marquis and Doris Dowling as the servant-girl at the chateau, who has helped him murder his first wife, so that she can become his second.
By contrast, the plot is even weaker than usual, resting on a dubious device about the blank space where the dead wife's portrait used to hang, and a mysterious stain that keeps appearing and reappearing, to haunt the conscience of the Marquis, but apparently not his new bride.
This brisk operator carries total conviction as the determined social climber, no longer just Charlotte, but Madame la Marquise if you don't mind, making changes to the house in line with her new status. "Some of this furniture is rather old" - "Not old, my dear. Antique." We immediately detect an uncomfortable culture-clash, as well as a personality-clash between the odd couple.
As for Max Adrian, he brings a lifetime of classical training to the role, as well as something of his native Kilkenny, in Ireland's deep south, where the ghosts and the goblins have not quite disappeared. His emotive speech, confessing all to the priest, is thoroughly Irish, despite the French-accented English in which the lines are meant to be spoken.
At the mid-point of the episode, there is a formal party, where the Marquise is playing the grand hostess for the first time, but this is only referenced, not shown, which upsets the flow. Two other weaknesses in the production seem to be perennials, recognisable from several other episodes. Both characters are seen at the piano, clearly faking it, when this is a standard piece of theatrical business: conceal the hands and move the arms in rhythm. And your host John Newland talks about strange things happening on the anniversary of the murder, which spoils the effect of his usually quite plausible commentary.