In the control room at 12m 36s there is a 10x zoom into a background character followed by a -5x zoom back to the Straker character, who is now in extreme closeup. The second focus pull is made whilst the lens is still zoomed in. and the zoom out happens after the focus pull is complete.
Both moves require skilled focus pulling, and are rare in TV work.
While the sixth episode to be broadcast, it was the fifteenth episode filmed.
The post and its arms holding up the balcony in Croxley's house resemble the Greek letter Psi. This letter is sometimes used in parapsychology as a symbol for research into extrasensory perception (E.S.P.).
The original script had the climax differing; when Foster arrives in the driveway of the ruined house (containing Croxley holding Straker and Freeman at gunpoint), Croxley senses his arrival, and shoots him in the shoulder. The wounded Foster takes cover, and the sound of the gunblasts as Croxley opens fire collapses part of the house, half-burying Straker and Freeman. Foster shoots Croxley dead, then rescues his comrades as the rest of the house collapses.
Two unseen radio operators and Shado man Stanley McGeagh voiced by Robert Rietty.