Season 17 ends on the 12th of January 1980, a good 6 weeks earlier than expected, due to the gap left by the unaired "Shada". Season 18 begins on the 30th of August, making this 33-week gap the longest Doctor Who has been off the air since the show began.
At first glance it looks like absence hasn't made the heart grow fonder, and not only does the first episode of Season 18 start with a disappointing 5.9 million viewers, this figure steadily declines to 4.5 million by the end of the 4-parter "The Leisure Hive". The rest of the season sees comparable viewing figures, with the second part of "Full Circle" a definite low point at 3.7 million. Viewers hover around the 4-5 million mark for most of the first half of the season, with no episode even reaching the 6 million mark, until fan favourite "Warriors' Gate" finally sees viewers of 7 million and more. All in all though, the ratings for Season 18 are decidedly down on Season 17. Even the 4th Doctor's swan-song, "Logopolis" fails to reverse the trend and Season 18 ends with its highest audience a mere 8.3 million for part 3 of "Warriors' Gate".
At an average of 5.8 million viewers across the whole 28 weeks, Season 18 is the lowest-rated Doctor Who season ever, ironically following the highest-rated season ever. What on Earth can have gone wrong?
Fandom argues constantly about the reasons for this, citing poor writing, a change in tone, even the fact that Tom Baker, the most popular Doctor ever, was leaving. However once more we must point out that Doctor Who viewers are not all Doctor Who fans, and if something better comes along, a lot of them will leave the show without a second glance. In 1980 that "something better" was "Buck Rogers in the 25th Century". ITV placed the flash, action-packed, post-Star Wars US import directly opposite Doctor Who on Saturday evenings, and completely slaughtered it in the ratings.