Season ten saw the continuation of a period of unprecedented behind-the-scenes stability on Doctor Who. It was the fourth successive season on which Barry Letts had worked as producer - a stint unequalled by any of his predecessors - and the sixth for Terrance Dicks as a member of the production team. The series' regular and semi-regular cast also remained unchanged for the third year in a row.
One change that Letts wanted to make was to introduce a new version of the series' theme music, arranged by Brian Hodgson and realised on the Radiophonic Workshop's Delaware synthesiser. This proved unpopular with others at the BBC, however, and so at a late stage the familiar Delia Derbyshire arrangement was reinstated.A policy that the production team liked to pursue was to launch each season with a 'gimmick' that would attract publicity and entice viewers to tune in for the start of the new run of episodes. For season eight it had been the introduction of the Master, and for season nine the return of the Daleks. This time, it was a special story to celebrate a significant landmark - the series' tenth anniversary. The lifting of the Doctor's exile by the Time Lords was a logical culmination of the policy pursued by Barry Letts and Terrance Dicks over the previous two years of gradually loosening his ties with Earth and allowing him once more to travel through space and time (albeit initially only on predetermined missions). It meant that the original rationale for his joining forces with UNIT was no longer valid, and the viewer was left to assume that he now maintained his association with the organisation through choice rather than necessity, perhaps because of the friendships he had formed with the Brigadier, Jo and the others.
At the same time, behind the scenes, the death of Roger Delgado and the departure of Katy Manning marked the beginning of the end of the period of stability that the series had enjoyed since Terror of the Autons; and further, more significant changes were looming on the horizon.