Season 18 The eighteenth season of " Doctor Who " ran between 30 August 1980 and 21 March 1981 . It was Tom Baker ' s final season . It was also the first season of John Nathan - Turner ' s producer ship . Many of the changes he brought in during this season would continue throughout the 1980s — some to the end of " Doctor Who " s original run . Amongst the more obvious changes during the season were : a new logo , a reworking of the theme tune , fully electronic incidental music , a raft of new writer s and director s , a new maroon - coloured outfit for the Doctor and the introduction of the question mark motif on the Doctor ' s clothing . Finally , it was Barry Letts swansong on the programme . He had returned for this one year to executive produce . Overview . Season 18 consisted of seven stories and twenty - eight episodes , making it the longest season in terms of episode count since the start of colour production . It was the last season of the original series to have an autumn - to - spring broadcast schedule and the last to have an autumn start until Season 23 . After this season , the number of episodes per season began to decline , after a relatively stable episode count for the whole of the 1970s . From " Full Circle " onwards , the season featured a story arc involving E - Space and the return of the Master . The stories were darker and more adult in tone than those during the Graham Williams era . They also attempted to emphasise science , much to the satisfaction of both script editor Christopher H . Bidmead and executive producer Barry Letts . ( REF : " The Fourth Doctor Handbook " ) Notably , this season revolves around the theme of entropy , linking together all of its stories in a rare phenomenon for the show ( while other story arcs would be linked together via people , objects , etc , the use of an overarching " theme " for a season was unprecedented ) . This theme would eventually foreshadow and culminate in the Doctor ' s regeneration at the end of the season . From a ratings standpoint , it sits right at the very nadir of " Doctor Who " history . It is easily the worst - ranked season of " Doctor Who " , with 17 of its 28 episodes outside of the top 100 shows . Indeed , the 9 lowest - charting episodes in " Doctor Who " history come from this season . " Full Circle " ' s second episode remains the worst - charting episode of " Doctor Who " . Its paltry 3 . 70 million viewers were only good enough for what was , even by the season ' s low standard , an extremely anaemic 170th place during the week of 1 November 1980 . In terms of pure ratings , as opposed to ranking , it was not quite the worst of the lot , with all four seasons after the 1985 - 86 hiatus faring worse in this regard . But it was easily the least - watched Tom Baker season , with an average of only 5 . 8 million Britons watching each episode . ( REF : " The Fourth Doctor Handbook " ) The decline in ratings has been attributed to direct competition from ITV . The first half of the season was up against , an American import with considerably better production values . ( INFO : " Full Circle " ) Adaptations and merchandising . Home media . DVDs . All serials of season 18 were released individually between 2004 and 2011 .