Cosmo asks Neville, "What is the law, Neville?" and Neville replies, ritually, "Do as thou wilt is the whole of the law." Thisa is a reference to Alister Crowley's black magic rites from the 1930s.
This episode was the first filmed with Honor Blackman and originally had additional footage introducing Cathy. However, as broadcast was delayed until later in the season this material was cut from the final print and, to some extent, replaced with new footage. It's fairly clear that the first words between Cathy and Steed were chopped out at the 6:31 (6:25) mark, and new footage is inserted until there's another sharp cut at 8:43 (8:35) where they return to the original take. There's another cut at 48:47 (47:30) as they discuss the case in the pub. Honor says, "Thanks, I needed that" and Macnee chuckles in reply when the splice comes in. She then says, "Did you ever find out who Gallion was working for?" (however, this cut is less apparent in the restored footage, and may be because of distortion being cut in previous releases).
Cathy Gale says her birthday is October 5, 1930, and that she believes it was a Sunday.
This is one of the few episodes that includes a Horror theme of any kind.