Why Every Classic Companion Left Doctor Who
18. Nicholas Courtney - Brigadier Lethbridge-Stewart
Nicholas Courtney first appeared as Colonel Alistair Gordon Lethbridge-Stewart in 1968's The Web of Fear. However, he wouldn't become a fully fledged series regular until 1970, when a new Earthbound, budget-saving format was devised by outgoing producer Derrick Sherwin.
Rather than lose this when he took the reins, Barry Letts and script editor Terrance Dicks instead doubled down on it, giving us the much-loved UNIT family that influences the show to this day.
When Letts, Dicks and Pertwee all left the show in 1975, the UNIT format was ultimately wound down with Terror of the Zygons. Courtney went on to refocus on his stage work, but he never fully left Doctor Who behind - returning in the 1980s alongside the Fifth and Seventh Doctors.