The eighteenth season of British science fiction television series Doctor Who consisted of seven four-episode serials broadcast from 30 August 1980 with the serial The Leisure Hive, to 21 March 1981 with the serial Logopolis. The season is Tom Baker's final as the Fourth Doctor before his regeneration into the Fifth Doctor, as well as Lalla Ward's as companion Romana II and John Leeson's as the voice of K9. For the second time, the entire main cast changed over the course of a single season. The season also sees the debut of Matthew Waterhouse as Adric, Sarah Sutton as Nyssa, and Janet Fielding as Tegan Jovanka, the three of whom would remain regular companions into the Fifth Doctor's era, as well as the return of the Master, portrayed both by Geoffrey Beevers and Anthony Ainley.