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In page Doctor Who:

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Doctor Who originally ran for 26 seasons on BBC One,[a] from 23 November 1963 until 6 December 1989. During the original run, each weekly episode formed part of a story (or "serial")—usually, but not always, of four to seven parts in the 1960s and early 1970s, four to six parts in the mid-to-late 1970s, and two to four parts in the 1980s.[citation needed] Occasionally, serials were loosely connected by a story line, such as season 8 focusing on the Doctor battling a rogue Time Lord called the Master,[1][2] season 16's quest for the Key to Time,[3] season 18's journey through E-Space and the theme of entropy,[4] and season 20's Black Guardian trilogy.[5]