After a 7 week break (23rd July to 10th September 1966) Doctor Who begins its 4th season with...the same Doctor. During this year Britain has entered the Swinging Sixties with a flourish, and the Doctor's new companions Ben and Polly are in part an attempt to reflect this and move the series with the times. So why do the first 4 weeks of the Season find ratings even lower than they were at the end of Season 3? Well it could be the decision to start with a somewhat pedestrian historical story set in 17th Century Cornwall, and the fact that the next story (set 20 years in the future and featuring spaceships, mysterious roving planets, and gleaming silver robots) coincides with a 1-3 million boost in viewers might indicate there's something in that theory.
9 weeks later however the viewers drop to about 5.5 million, which is unusual since it coincides not only with a Dalek story, but a Dalek story that would have been publicised in advance...and was entitled "The Power of the Daleks".
Of course as any self-respecting Doctor Who fan will tell you, this was the moment when the series made an incredibly bold move, unique and groundbreaking for a TV drama series. The leading man was not only recast, but the change in actor was acknowledged on-screen, and became an integral part of the story. We will never truly know what the majority of viewers thought of this change, but it's a fair bet that the audience was divided into those who stayed with the programme, those who turned off at the thought of losing "their" Doctor, those who tuned in to see how the new guy got on, and those who didn't care so long as it had Daleks in it. Whatever the reason, the new Doctor's first story was watched by 5.5 million viewers. That's 2 million less than his predecessor's swan song, but just over 1 million more than had watched "The Smugglers" 8 weeks earlier.
Had the new Doctor revitalised interest in the show? Possibly but if so, not by much. Figures hover around the 7 million mark for most of the rest of this Season's 43 weeks, and yes there is a dip in viewers for "The Highlanders", though whether that is because it takes place in 18th Century Scotland, or because it follows the Daleks, who can tell? For the first time in Doctor Who's history the Daleks appear twice, once to herald in the new Doctor, and once to close the season with a Dalek spectacular that fandom regards as a lost classic, but strangely does worse in the ratings than the rest of the season, its middle episodes scraping the 5 million mark. The new Doctor's first season ends with his last two episodes watched by under 6 million viewers.