When illness forced Hartnell to leave the show in 1966, the canny concept of regeneration allowed the character to transmogrify into Patrick Troughton. The Cybermen (below) were promoted to the position of enemy-in-chief while Dalek creator Terry Nation attempted to launch a spin-off series in America. 1970 saw the series broadcast in colour, and marooned Jon Pertwee's flamboyant Doctor on Earth as a scientific adviser to the UN.
The show's popularity peaked during Tom Baker's tenure, when the series flirted with Gothic horror and incurred the wrath of Mary Whitehouse. The nastiness was toned down, K9 introduced, and the show became more overtly comic. After Baker's departure in 1981, Peter Davison played the Time Lord as a breezy, vulnerable figure.
Colin Baker made the role more boorish, and his stories saw the horror element return - a scene of rat-eating came in for particular criticism. BBC1 Controller Michael Grade took Doctor Who off the air for 18 months in 1985 and on its return, the 14-episode epic The Trial of a Time Lord reflected the programme's uncertain future.
Three more series, starring Sylvester McCoy, were produced, but audiences continued to shrink and the programme was eventually exterminated in December 1989.