Trivia: "The Three Doctors" was the last appearance of William Hartnell as the First Doctor. The show was written to include all three actors that had played the role as a way of celebrating the show's 10th anniversary. However, by the time the show was made, William Hartnell was in failing health (due to the debilitating ailment, arterial sclerosis). Because Hartnell was so frail, he could only play the part of the Doctor seated in a chair, reading his lines from cue cards. (The plot device of Hartnell being "caught in a time eddy" was written in to explain why Hartnell was only ever seen in the story on TV monitors). Also due to Hartnell's failing health, his contribution to "The Three Doctors" was limited to one day's filming (Monday 6th November 1972) at the BBC's Ealing Studios. "The Three Doctors" was also the last time Hartnell did any acting: he died two-and-a-half years later, on 24th April 1975, aged 68.