Doctor Who 1963 - Season 10, Episode 26: The Green Death: Episode Six
This features Jon Pertwee as the Doctor and also Katy Manning as companion Jo Grant. Nicholas Courtney returns as Brigadier Lethbridge-Stewart. In order to celebrate the tenth year of the show both William Hartnell and Patrick Troughton returned as the First and Second Doctor. 'The Three Doctors' is the first Doctor Who serial in which an earlier incarnation of the Doctor returns to the show.