The Keeper of Traken is the sixth serial of the 18th season of the British science fiction television series Doctor Who, which was first broadcast in four weekly parts on BBC1 from 31 January to 21 February 1981. In the serial, the alien time traveller the Master (Geoffrey Beevers) seeks a power on the planet Traken known as the Source which he wishes to use to restore his life. The end of the serial introduces Anthony Ainley as the Master. The serial is also the debut of Sarah Sutton as companion-to-be Nyssa.