Ontario was a province of Canada. Toronto was a major city there. The Seventh Doctor once found himself there at Christmas. The province was also home to Lake Ontario. (ST: Christmas in Toronto) For unclear reasons, Sarah Jane Smith kept a vehicle license plate from the province in her attic. (SJA: Invasion of the Bane)
Behind the scenes
Ontario was home to TVOntario, one of the most faithful broadcasters of the 1963 version of Doctor Who in North America. They were responsible for commissioning Howard Da Silva's continuity announcements, which figured prominently on the early North American broadcasts of the Tom Baker era.
