William Hartnell's First Doctor ruled the airwaves in England from 1963-1966, when Patrick Troughton took over. Troughton stayed in the role until 1969, and at that point the series was in trouble. Adventures through time and space on a shoestring budget had become all too familiar to viewers, and audiences had dropped off sharply, despite the brilliance of its leading man. Additionally Troughton's final season was plagued by behind the scenes woes leading from the punishing production schedule of making over 40 episodes per season. With the ever-changing and expensive futuristic settings to realize, slipping ratings and a concept that seemed to be getting tired, tough choices had to be made.
The show teetered on the edge of cancellation, but the production team came up with a novel idea; they would maroon the Doctor in London of the very near future, establishing him as being taken on as the scientific advisor for UNIT, an international paramilitary organization who investigated the odd and unusual. The series would now be broadcast in colour, and the length of seasons would be brought down to a reasonable 26.
Troughton moved on, and light comedian Jon Pertwee was cast as the Third Doctor. Initially it was hoped Pertwee would bring more comedy to the part, but he decided to play it totally straight, creating a Doctor that was a bold, arrogant and dapper man of action. Paired with fellow scientist Liz Shaw and working for the stalwart Brigadier Lethbridge-Stewart, the new concept cut costs, refreshed the series and soon was bringing back the ratings.
In this classic scene from the final story of Pertwee's first season, Inferno, the Doctor finds himself transported to a parallel Earth, one where England is a fascist state and where he is confronted by sinister versions of the people from UNIT.