Consider ‘Time and the Rani’ – Pip and Jane Baker (may they rest in a slumber of concord and amity) were commissioned to write it before Cartmel was hired, because Nathan-Turner knew they could write something quickly. They were the last vestige of the previous version of the show, and you can see a clear divide in the concerns of ‘Time and the Rani’ and the rest of Season 24, and from there the rest of the McCoy era.
‘Time and the Rani‘ shares the CBBC tone of the rest of Season 24, but otherwise it’s an adventure yarn about an amoral scientist on an alien planet. The rest of the series’ stories are grounded in some real-world concern, but still cut from similar pulpy froth. This is Doctor Who mashing up genres, something it hadn’t done consistently since the mid-Seventies. Season 25 abandoned this approach and moved back towards adventure stories, but added enough substance and depth to be an improvement. Only ‘Silver Nemesis’, where the substance of the story is the show’s own mythology, really falls flat.
Season 26, the final series of the original run, integrates these approaches: genre mashups aiming for depth and complexity, grounded by focussing on the companion from present-day Earth. The CBBC tone of Season 24 is one of several styles, and stories continue to address the show’s mythology in more interesting ways (the interrogation of British history and attitudes, for example, and the line in ‘The Curse of Fenric’ about the Doctor not knowing what’s happened to his family). In three short series Doctor Who has gone from the kind of show that, on a bad day, produced ‘Timelash’ to one where a bad day produces ‘Battlefield’. Furthermore, it’s developed with to a point where it can be favourably compared with Russell T. Davies’ approach for 2005’s relaunch.
That is Doctor Who changing slowly. It’s capable of even more rapid change than that, such as the shifts between Seasons 7 and 8 and 17 and 18.
While Barry Letts and Terrance Dicks made most of Season 7, the Earthbound approach and longer story lengths were the work of outgoing producer Derrick Sherwin. This approach borrowed heavily from ‘Quatermass‘, and the new production team were keen to change things for their first full series. As a result Season 8 has more and shorter stories, a change in companion from a scientist to a novice, and moves away from the dominant gritty tone in the last series. Season 8 mixes things up with an increasing cosiness and occasional bouts of psychedelia interrupting the action by HAVOC. Season 7 acts as a buffer, so the distance between the Sixties and Seventies isn’t as jarring as it could have been.