Three geniuses meet.
This is a promotional photograph taken in early 1972 for the Doctor Who tenth anniversary special, "The Three Doctors". From left to right, ti's Patrick Troughton, Jon Pertwee, and William Hartnell. It was the only time the three actors were together.
William Hartnell agreed to appear in the special despite having acted only three times since leaving Doctor Who in 1966 and only once since 1968. He suffered from high blood pressure, effective treatments for which did not appear in the U.K. until 1968. Untreated high blood pressure causes heart disease, stroke, and memory loss. By 1966, Hartnell was suffering from minor to moderate memory loss... by 1970, he had congestive heart failure that left him wheezing, weak, and unable to stand for more than a few moments.
Hartnell's wife contacted the producers of Doctor Who and informed them of her husband's medical condition. Although the original script had him playing a full role alongside the Second and Third Doctors, he couldn't do it.
Instead, his role was reduced to a pre-recorded one, where he sat on a chair in the BBC's Ealing Studios while being filmed from the head and shoulders up.
It was the last time William Hartnell played the Doctor, and his last acting role. In December 1974, he was admitted to a nursing home. He suffered a series of strokes, and died in April 1975. He was just 67 years old.