The sixth season, as the last to be made in monochrome and the last to feature Patrick Troughton as the Doctor, marked the end of an era for Doctor Who. It was also made during a rather unsettled period for the series behind the scenes and, perhaps as a result of this, lacked the consistency of the previous season.
It did however contain a number of stories that would later come to be regarded as particularly fine examples of sixties Doctor Who, and three of these - The Seeds of Death, The Invasion and The War Games - contained elements that, with hindsight, can be seen to have foreshadowed the revamping of the series that would take place for the first season of the seventies.
The production of season six had proved generally problematic. Of particular significance was the fact that during this period both Peter Bryant and Derrick Sherwin became involved with other projects - notably the military drama S P Air, a two-part pilot for which was actually made and transmitted - and so were unable to give Doctor Who their undivided attention.
The Dominators went out under the pseudonym Norman Ashby after writers Mervyn Haisman and Henry Lincoln objected to Sherwin's rewriting of it, including its reduction from six episodes to five. The Mind Robber, which had at one point been planned as a six episode story itself, was ultimately lengthened from four episodes to five in order to fill the extra slot.
Perhaps even more significantly, three of the seven transmitted stories - The Krotons, The Space Pirates and The War Games - were late additions to the season after a number of other stories, including Dick Sharples' The Prison in Space, Paul Wheeler's The Dream Spinners, Malcolm Hulke's The Impersonators (which may have evolved into The War Games) and one by Derrick Sherwin of unknown title, had for one reason or another been abandoned.
The season was nevertheless more varied than the previous one not only in terms of quality but also in terms of settings and plots, eschewing its heavy reliance on Earth-based stories and the 'isolated group of humans infiltrated and attacked by alien monsters' formula. This time there were only three stories - The Invasion, The Krotons and The Seeds of Death - that could really be considered traditional monster tales.
This however was more a matter of economic necessity than an artistic decision. The production team continued to amortize the series' costs as far as possible by commissioning relatively long stories, but found that the budget would simply no longer stretch to the creation of large numbers of convincing alien costumes and environments (or even of much incidental music - hence the dearth of this, particularly in the first few stories). They had in any event concluded at an early stage of the season's production that Doctor Who was no longer working in its current format and ought to be revamped.
Inspired by Nigel Kneale's highly successful Quatermass serials of the fifties, in which Professor Quatermass and his scientific and military colleagues had been seen to battle a succession of alien menaces in near-contemporary England, they felt - based in part on the perceived success of The Web of Fear, a story that had itself been somewhat influenced by the Quatermass serials - that the Doctor's adventures would be far more effective if they became less fantastical and took place on Earth in familiar, everyday settings with recognisable characters.
To facilitate the remoulding of Doctor Who in this image, Sherwin created UNIT - the United Nations Intelligence Taskforce. This was to be an international military intelligence unit, established to investigate UFOs and other strange phenomena, with which the Doctor could work while on Earth. Having liked the character Colonel Lethbridge-Stewart in The Web of Fear the production team decided, subject to actor Nicholas Courtney's agreement, to bring him back as the commander of the British branch of UNIT.
UNIT's debut in The Invasion, with Lethbridge-Stewart promoted to Brigadier, was thus always intended to be simply the first step in a process of moving towards a more permanent Earth-bound setting - something that would have happened even if Patrick Troughton had not made clear his intention to leave at the end of the season.
By the time of the making of The Invasion, Sherwin was effectively deputising for Bryant as producer of Doctor Who, leaving Dicks to take over as script editor (as the story editor post had now been renamed). A young writer named Trevor Ray was meanwhile brought in to replace Dicks as assistant script editor. Bryant himself was becoming less and less actively involved with the series due to health problems. His last credit as producer was on The Space Pirates, for which Sherwin temporarily returned to script editing duties while Dicks was busy rewriting episodes of The Seeds of Death. The producer's credit on The War Games then went solely to Sherwin.
The conclusion of The War Games, with the Doctor being captured by his own people and sentenced to a period of exile on Earth, was specifically designed to usher in the new format that Sherwin and Bryant had devised for Doctor Who. The next season - following an unprecedented six-month break during which the series' slot would be occupied by a debut package of episodes from the American import Star Trek - would see that new format finally coming to fruition.