Our third story featuring the War Doctor had it's genesis in an idea for a regeneration story for the Saxon Master. Upon the appearance of Missy in Dark Water, we immediately began looking to see if we could craft a flashback story to show how she was born.
While the footage featuring Michelle Gomez really popped, we couldn't find anything appropriate for John Simm, and so the story (provisionally titled "The Night of the Master") was abandoned. And a good thing too, as later, "The Doctor Falls" would go into more detail about the birth of the Missy incarnation.
When we began work on Season 26C, the story idea resurfaced. The existing footage featured Gomez playing "an immortal" and referenced events stretching back many hundreds of years. While no time travel was involved, I believe we fudged the issue enough to show a few scenes on early Gallifrey.
The John Hurt footage took much more digging to find until we finally settled on a TV film concerning politics. More than any other story in our series, this was a Doctor-lite one. However, with references to a former companion of the Doctor, along with a cameo by said companion, we believe we have done a decent job at showing the War Doctor pulling the strings in a secret battle with his archenemy: The Master.
This episode uses footage from the following Non-Who sources:
Highlander: The Raven (Inferno)