Because of a BBC strike, this story was shot entirely on location with no studio scenes. This made it the first Doctor Who (1963) serial to be made entirely on location and the only serial that was ever made entirely on film. The serial came close to the brink of being canceled after the first week of filming, but producer Derrick Sherwin persuaded the BBC to complete it on location. As a result, this serial was shot in about six weeks between September and November 1969 rather like a low-budget movie. Director Derek Martinus said Sherwin was "a very energetic and determined bloke. He had a tremendous fight to get the go-ahead, but he did and for a while, we all had this wonderful fantasy of doing Doctor Who all on film and selling it to America."
This show marks two important changes to the broadcasting of the series: It was the first show to be shot and transmitted in colour (as are all subsequent shows), and the number of shows made per season was lowered from 40 to 45 to approximately 26.
During the Doctor's medical examination, a unique point of Time Lord physiology is established: they have two hearts.
Because it was made entirely on film, in 2013 this was the first Doctor Who (1963) serial to be released on Blu-ray disc.
The working title of the serial was Facsimile and was based on a story that Robert Holmes wrote for the British science-fiction film Invasion (1966), which featured an alien crashing in the woods near a rural hospital, where a medical examination reveals his alien nature. The hospital is later visited by other aliens seeking a fugitive criminal. Some of the exact lines of dialogue used by human doctors to describe the physiology of the injured alien were re-used.